The divisions between the two sides in terms of actual concrete policies was a bit less clear cut than we would think. Some of it was just interpersonal or for reasons for family affiliation.
But the Populares generally at least were in favor of reforms that increased the land and employment opportunities for Roman citizens and freemen who had crowded the cities and were living off of the dole. They also however were made up of more ambitious and (in many cases) personally greedy men, who were less respectful of the traditions of the mos maiorum and were far more tolerant of bending the existing rules.
The Republic was structurally rotten and someone was going to break it in some manner. The Populares would have broken down into infighting had they taken control, as Gaius Marius wasn't sufficient in keeping them together, while the true believer element of the Populares (who wanted land reform, more democratization at lower levels, public works projects) were powerless without a strongman protecting them. The Optimates would have tried to continue without meaningful reform until riots began to sap away at the rule of law and another aristocratic favoring strongman would take control, this time for good.
I admit my knowledge of this to be somewhat lacking, so a good source would probably be Mike Duncan's book on the end of the Republic. The Storm Before the Storm: The Beginning of the End of the Roman Republic just came out and I've gotten through a bit of it. But it is probably a good source for this.
I just finished
The Storm Before the Storm which is part of why I started up the WI. I also think that while both parties had similarities and that there was at least as much interpersonal and factional strife involved in the division as anything else. There also seem to be quite clear-cut and concrete differences between what they viewed as the end goal. I am honestly not sure that the Populares would break down to the same degree as the Optimates did post-Sulla. While there are strong personalities in the faction, as long as Cinna was present he seemed remarkably able at controlling his faction. His death really screwed the Populares, who lost their leading figure and had to work through men like Carbo who simply weren't as skilled as Cinna.
My main issue regards what three figures would do. Pompey, Lucullus and Metellus Pius. Pompey seems like the type who would demand honours far before his time, which the Populares would probably need to bow to in order to keep him quiet. What this would entail, I really don't know. Lucullus could very well take up Sulla's mantle but he simply doesn't have the same degree of support as Sulla, and as has been mentioned probably would end up with his forces mutinying. Metellus Pius was in Africa at this point and was planning to meet up with Sulla, but with Sulla dead and the Optimate position collapsing he probably either comes to terms with the Populares or is crushed. By this point the actual bloodshed had been rather minor without any significant military clashes between Romans, so with the Populares interested in just moving past this bit of trouble they might agree to let Pius back into the Senate. The question becomes whether Pius would be willing to accept this.
Gaius Marius was always more of a burden on the Cinnan regime (except for his initial role in securing Rome for Cinna) than an aid, and his death actually strengthened Cinna's grip on power significantly.
I don't think it would have mattered in the long run, the Republic was already in the path to ruin.
The fall of the republic started back when the Senate murderer Tiberius Gracchus in 133BC. Then, some 50 years later, Marius Jr. and Sulla were locked in deadly combat. Marius Sr. set the stage for the rise of the Empire by removing the property qualification necessary to serve in the army. When Marius Jr. became the army's paymaster he transferred the soldiers loyalty to himself. If Sulla loses, things will probably continue on the same path. Sulla's second war ended in 82BC and only 10 years later Crassus, Pompey and Caesar were working together, soon to form the First Triumvirate. All three of them had a thirst for power. Ceasar bet Pompey after Crassus died and the republic was history.
The death of the Republic was destined by the weakness of the Senate and their own corruption. All it took was the right man at the right time to take power.
At most, the POD can buy the Republic time.
Still, thats not what you're asking is it?
Well, I'm not an expert, but there were a few thing the populares were most well known for wanting. Among those are:
-They supported laws regarding the provision of a grain dole for the poor by the state at a subsidised price.
-They wanted reforms which helped the poor, particularly
redistributing land for the poor to farm, and debt relief.
-At times they also supported the extension of Roman citizenship to Rome's Italic allies.
From those, I recon the third would be the one with the most profound effects. As to what those effects would be, I really don't know.
In the long run the Republic either needs something that fundamentally turns back the clock (though Sulla tried that and it failed, so probably wouldn't be able to do that) or the Republic needs to change significantly. The question is more about what that end goal will look like. I don't think that Augustus' combination of Tribunate, Consular and Pontific powers in a single person was the obvious end goal. There are a whole range of directions things could have gone in. Furthermore, escaping Sulla's proscriptions and the ban on the proscribed's families on political office - and all the other shit that followed his reign, would remove a lot of the bitterness and willingness to discard norms left and right. You certainly already had a breakdown of Mos Maiorum at this point, but Sulla's defeat would prevent him from becoming a model for later figures. Without the reestablishment of the Dictatorship, Sulla's proscriptions or his ability to act completely without any legal power and still win, there aren't going to be the same set of precedents in place. The path to power will look significantly different - what it would look like is what I am trying to figure out.
The extension of Roman citizenship to the Italic allies had already been agreed by the end of the Social War, it was more a matter of how those votes would be distributed between the voting tribes. I actually think this is where we would see the least shift from OTL, because a major part of Sulla's settlement was the inclusion of the Italics evenly distributed between the rural tribes. The Italics will have far more influence compared to previously but not much more than they got IOTL. The major difference will be that the Optimates, or at least those who succeed them, won't have clearly established their support for the Italics, which Sulla did IOTL. Thus, the Italics are probably going to continue supporting the Populares, resulting in a significantly stronger Populares faction which would be able to call on the Italics as needed. This means that the Optimates likely won't be able to gain power anywhere near as easily and leaves the only relevant faction (outside of coups, revolts etc.) as the Populares. All political struggles will therefore likely occur within the Populares sphere of politics.
The grain dole probably continues to play a major role. The Equestrians probably keep their position in the Juries, instead of it returning to the senate as per Sulla, while their Publicani companies probably gain more power over the provinces - leading to even more rapacious tax farming. Whether land distribution even works as a solution to the poor I don't know, I don't think that it was possible by this point to prevent the Latifundi economy, but establishing roman colonies in conquered lands would probably still be a possibility. This could actually serve as a driver for expansion, with popular demand for land driving the expansion.
Cinna definitely remains top dog at this time. It looks like he liked to do some important jobs himself, but he would probably leave years-long generalship overseas to more capable men (who are loyal). Quintus Sertorius seems like a great choice to head east against Mithridates, and that's another point hugely in the fledgling popular Republic's favor: there's only one front of conflict, against Mithridates, not the second front, against Sertorius in OTL. I can't see Lucullus making a settlement with the populares, even with Sulla dead, but I can definitely see Lucullus's legions mutiny or else be destroyed by Mithridates and the Republic. Mutiny is most likely I think, and then Sertorius will have plenty of legions to kick ass in the east.
On the home front Cinna is, as I said, top dog, no matter how much Gaius Marius Junior's name is worth. The young man will be at the forefront always, his father's popularity guarantees that. While hotheaded, I think he can be trusted to trust Cinna's program for Rome. First and foremost is citizenship for all the Socii. The populares dominate Rome now, with popularity from pretty much everybody third class and below who's not a veteran of Sulla's, and many in the second and first classes as well. BUT they can't just hand citizenship over to the Socii, thereby fucking over all the citizens supporting them by diluting their vote. There must be a settlement, and a solution which pretty easily comes to me is that the centuriate/people's assembly will be weighted more fairly (the first census class already had over 50% of the centuriate votes), and in exchange the Socii and Latins become citizens and are of course enfranchised.
Now what to do with the tribal assembly? I thought of having Italy divided into regions and each region makes up a tribe, but that idea seems too modern and also putting too much power in the hands of the only recently Roman Italians, which will decrease support for the law among Romans. Then of course there's distributing new citizens equally among the tribes instead of stuffing them into the urban tribes. I can see this being done without too much difficulty for the new citizens who were recently free Italians, but not for any slaves who will be freed. Or else the tribal assembly can remain the same.
Any changes in the tribal assembly will radically alter the plebeian assembly, which is basically just the tribal assembly without patricians, but swung around a lot more political weight. I can see the centuriate and tribal assemblies being used mostly for elections, and the plebeian being where most of the action is. Despite Cinna, a patrician, leading Rome at this time, it would be in keeping with the main thrust of the rest of his political agenda. A lawmaking patrician consul or praetor will have to go through an ally or henchman plebeian tribune.
Patricians don't get the two year head start in the cursus honorum that Sulla gave them, but that's no big deal. Sulla's tabularium isn't built, and I feel as though the populares will want to sweep everything that besmirches their name under the rug, so there will still be a kind of lax view on record keeping (of current events, deaths, state events, etc.) except as regards to laws. The grain dole and tribunician powers and sacrosanctity are set in stone.
There will probably also be a law ensuring that soldiers get their ten or twenty or whatever iugera of land after serving in the legions. Marius was big on it and I'm honestly surprised that Sulla didn't work more on making it a reality, since it was clear that that would keep the legions more loyal to the Senate/state rather than their commander.
Caesar being the son-in-law of Cinna and the cousin of the son-in-law of the Pontifex Maximus Quintus Mucius Scaevola, and his own stubbornness, will probably get him to avoid being flamen dialis. His connection with the wealthy and surviving (as they were moderate and, with no Sulla, had no reason not to support Cinna) Cottae is nothing to sniff at either.
Now, not all of the aristocracy will support this pretty radical (for this time) political program. There will be a small, probably always kind of in danger, faction in the senate who'll nevertheless be able to use their influence to temper the most extreme ideas. In particular I expect them to block the change of the tribal assembly, and to moderate the redistribution of the centuriate vote weights. This doesn't mean that its individual members won't be illustrious. Cinna was ruthless but he avoided unnecessary bloodshed and I don't see him exterminating all optimates. Without Sulla they are leaderless, except for maybe the pontifex maximus, who will succeed in keeping the political program moderate because Gaius Marius Junior is his son-in-law, and will also avoid any repercussions because of that connection. The fact that Marius Junior had him killed IOTL is immaterial, it was a very desperate time and any influential person (and the pontifex maximus is very influential) in Rome who could and would help Sulla was an existential threat. Not justifying his murder, just saying that it wouldn't happen ITTL.
I agree that Sertorius going east would be awesome, but I don't know if he is senior enough for it. His later role in running the Hispanic resistance has really boosted his position, but at this point in time it could as easily be one of the consuls is sent to take up command, or that Cinna himself does so. Then again having Sertorius as commander, with Gaius Marius - maybe as Praetor and Caesar coming along as military tribune would be awesome. I largely agree with you that Lucullus probably takes up command after Sulla but doesn't have the prestige to keep them from mutinying.
I don't think that Gaius Marius the Younger could have become Consul without the emergency situation. He could probably make the leap to Praetor and go off with Sertorius to serve in the east. That would remove a significant rival to power for Cinna, while still coming across as a boon. As I have mentioned above, the largest difference from OTL in regards to the Socii will probably be that the Optimates haven't accepted them like Sulla did. Sulla's decision to back Socii voting rights resolved that issue, but without him to do so it is still only the Populares backing them which drives them into Populare hands. I think that distribution the Socii across the Rural tribes equally, as they did IOTL and as Sulla kept in place, probably ends up being their solution.
I agree that the Assemblies will probably receive significantly more power, while the Tribunes remain a jumping off point for successful careers as they were before Sulla's reforms. I think we might see more people follow the tribune route to higher office and the Assemblies become the most important building block. My question is what happens with the Senate, how do they retain power, because I doubt that someone like Cinna would accept too much of a loss of power to the Assemblies. There has to be some form of balancing mechanism.
I am also not fully certain that the sacrosanctity would be maintained, or at least officially enshrined, because of worries that someone might abuse that power or turn in against Cinna. Keep in mind that the Populares now control the Senate, so they might have some interest in retaining some sort of power through that body. Could you explain why record keeping would be more lax? I hadn't heard anything about that before.
Sulla actually did distribute lands to his men in Italy, but most of it ended up in the Latifundi anyway. By this point it seems unlikely that the small-holder will be retained in Italy itself. That is why I think that establishing more colonies abroad would probably see more support.0
I am pretty sure that Cinna was the one pushing Caesar as Flamen Dialis. I think that without Sulla directly stripping him of the position, it will be difficult to escape it. Even so, I think that given his close connections to the leadership, Caesar could probably find ways of working around it and embark on a political career despite still being Flamen Dialis. It would signal yet another breech of Mos Maiorum, but it wouldn't be the biggest so far. IOTL after he was stripped of the position, the other Flamen took up the Dialis' functions themselves. Caesar really is positioned to have a completely insane career if he can simply get around the issues of the Flamen Dialis - though that will be very difficult. I guess keeping the position might be going a step too far, but I don't see Caesar being left out of a career.
I think that the program might actually be more moderate than we would expect, simply because Cinna would want to keep the moderates on side. The Optimates would probably be allowed to come crawling back, but I also think that Cinna would swamp the senate with supporters, further cementing his branch of the Populares' power. We see him doing this with Caesar's election as Flamen Diailis. Another consideration is who gets elected as Pontifex Maximus - this might actually be how Caesar gets out of the prohibitions on leaving Rome. The previous Pontifex had already broken the rules surrounding leaving Italy, so he would be able to continue a career as he did IOTL. Even if the Pontifex Maximus post was given to someone else, the breaking of the taboo surrounding leaving Italy could serve as building block to the Flamen Dialis leaving as well. Sorry if that was a bit confusing. I meant that if the Pontifex could break the movement taboo, then the Flamen Dialis might also be able to. That said, could a single person be both Flamen Dialis and Pontifex Maximus?
I really enjoyed all your answers, so I hope we can keep up this discussion.