With your new constitution you did some good first steps to reform the roman republic inside Rome. But unfortunately thats just half the battle. I like to say "Rome conquered an empire. And afterwards the empire conquered Rome". This process of converting a city state into a real integrated empire lasted over 300 years and is one the most important processes of roman history. So we have to moderate it in your alternate history.
Empire-wide processes and mechanisms are a wide subject. Therefore let us focus on Governors, Legates and Province Management in a first step, which will lead us soon to a reform of military and administration with heavy impact on parts of the constitution. Because for a roman there was no difference between politics, military, administration and even jurisdiction. No divison of powers outside of Rome. All was mostly done by one single person: the proconsul or legatus augusti pro praetore. Just finance-managment was separated later beginning with Augustus reign.
These governors where a serious issue during late republic and empire, because they trend
1. to corruption and illegal exploitation of their province, and
2. to usurp against Rome and/or the emperor if a chance for challenging was granted
There are some more issues with these guys like starting wars illegally for personal Honor & Glory as well as loot in late republic, and overly bureaucratizing the administration in late empire. But let's focus on the 2 most important issues above.
Corruption and exploitation
This corruption and explotation, which was common practice during the republic, had multiple reasons. First it was possible, and thats more than enough for greedy people. On the other hand the republican constitution downright enforced a governor to exploitation. He had to finance a very expensive election campaign to become praetor or consul. So he needed a payback urgently the year after as a governor. One measure here could be, to simply get rid of these elections by the plebs.
This exploitation was a heavy burden for the people onsite and the roman treasury. Additionaly overtaxation for the purpose of self-enrichment lead often to riots. It is not hard to assume, that the pannonian riot in 6 AD and the german riot against Varus in 9 AD was caused by overtaxation mainly. So just with a honest governor like Cicero in Cilicia or Plinius in Bythinia, the battle of Teutoburg-Forest would perhaps never happened.
I am convinced, that no central administration in Rome can stop this exploitation effectively. The special court in republican Rome for "actio de repetundis" was not effective. Even during principate exploits could not be fully stopped. And in late empire things became worse again.
Rome is simply too far away, longrange communication is hard in ancient times and corruption is everywhere. What would help, is more local control. But most historians are convinced, that the idea of federalism was so strange in ancient times, that it would never work, or worst case lead to separatism.
But perhaps a little bit of local control could be introduced, without boggling the mind of the romans. Augustus already introduced the "Consilium Provinciae", e.g. the Consilium Tres Galliae. Unfortunately these yearly conventions of delegates of the cities had no legal power. Their duty was the emperor cult and perhaps they grabbed the chance, to talk with the governor about measures informally.
Now let us formalize this council a little bit. Of course, it could never become a full blown legislative, because this would mean federalism. And just roman citizens in the provinces could become elected for delegate. Fortunately, most nobles of peregrine cities or pacified tribes had granted roman citizen rights anyways; not counting the native romans settling in the provinces.
So in order to control the governors we need a permanent local consilium provinciae with more legal power:
- control of the yearly budget plan of the governor, perhaps by electing a local provincial quaestor
- the right to veto against measures of the governor (not against central law or measures coming from Rome) via using the roman senate as court of appeal
Alternatively or additionally you could introduce permanent "Correctores". The emperors appointed correctores occasionaly and temporarily to deal with ruined regions, e.g. Corrector per Galliam, Corrector per Africa. So a corrector often covered more than one province like the late empire "diocesis". But with a much leaner organisation. These correctores could now also supervise the governors onsite. In coniunction with the consilii using the correctores as first court of appeal, the governors would have a much harder time to exploit provinces.
Perhaps veto against measures and financial supervising is too much for the roman political culture. In this case it would make sense to start with some weaker instruments and give the strong rights just to fully romanized provinces like Sicilia and Gallia Narbonensis first. However there should be a kind of local control, so governors become more careful.
Usurpation and Civil Wars
Now this part is much of a bigger challenge than avoiding exploitation. Roman emperors tried to avoid usurpation by giving every legate not more than 1 legion (Domitian). But this never worked, because in some areas this is strategically not acceptable. Also introducing a central big field army, which should secure the emperor, did not work, because exactly the leaders of these big armies usurped. And both models (single legion, one central field army) was detrimental for the performance of the roman army in general.
However some measures may help to reduce usurpations in a republic:
1. Military Academy
The romans were great fans of training on the job, with the result, that young officers were not qualified sufficently. Young tribunes came directly from roman high school, where they learned rhetoric and grammar mainly. Additionaly they hopefully had read some books about military and made some exercise for fitness. Thats it!
In the military academy the cadets learn about military and administration. Perhaps even more, but more about that in a next post about the educational reform of the republic and universities.
Additonally such an academy could swear young officers to the senate and constitution of Rome.
2. General Staff
The romans had no central general staff as a central high-command, which would have been able to control the commanders in the provinces at least by authority. Also the chance to become a member of the general staff could be a goal for legates, which otherwise have no further option of promotion (but usurp for the throne)
3. Roma Cult
The emperor cult helped a lot during the principate to avoid usurpations against the emperor. The legionairs simply refused to join an usurpation, even if their commanders were willing to do so. However, if the emperor was dead, that was a different story, even during principate. In the republican model, we have no dead emperor with an unclear succession, which otherwise would give automatically room for usurpation. It is clearly the senate, who elects the new princeps. Thats one reason why I strongly recommend to elect the princeps like every magistrate for 3-5 years, in order to avoid any inheritance, which just leads to royal trouble.
Now without a divine emperor, we should promote the Godess Roma to the primary God of the soldiers, with a statue and an altar in every camp. Roma ist not just a known godess, now she stands also for Senate and People of Rome (SPQR) and the holy constitution of the republic. The primary religious duty of every soldier becomes the protection of the constitution.
Additionally we have the already proposed Decemviri Militares( a committee of the senate), which adminstrates the aerarium militare, pay every soldier regulary, pay his donativa and pay his pension plus could decide finally about promotions from centurio upwards. This would help to buildt loyality to the senate, too.
4. Military career
The cursus honorum of a young son of a senator was a mix of political and military offices:
1. the start was usually the "Vigintisexviri", some lower grade officials in Rome, e.g. responsible for the streets (viarum)
2. without any further clue, the young noble applies for tribunus laticlavius, the deputy commander of a legion
3. Quaestor (now he becomes a senator officially)
4. perhaps another time as tribunus laticlavius, but just vir militares did that
5. Aedil
6. now our young guy could command a legion as legatus legionis but reporting to a legatus legionis pro praetore and governor
7. Praetor
8. now this guy became a governor and as legatus augusti pro praetore he could command more than 1 legions in some provinces. Mostly just 1 or 2 but if talented even more
9. Consul
10. mostly just ex-consuls commanded the real big armies of up to 4 legions, e.g. governor of Syria.
The problem with this cursus honorum is obvious to me. When a senator reached his maximum military power, he already had passed maximum political power, too. There was nothing rest. No goal, no carot, which could prevent adventures. Just sitting in the senate and discuss endlessly after returning to Rome.
One measure could be to split military and political offices and careers fully like Diokletian did later. But this lead to a very dangerous esprit de corps which promoted usurpation even further, because these officers now had no clue about adminsitration and showed the necessities of the government no understanding.
Therefore I support a career, where senators hold military and political appointments, but never in personal union. So a split of offices, but a combined career.
Lower Career
The young officer has to complete the new military academy first.
Afterwards the young officer has to hold one or more appointments as a tribunus militum and afterwards as an asistant of a proconsul, which is now a fully civil office.
Last station is deputy commander of a legion as a tribunus laticlavius and afterwards again a civil job, maybe as curator in Rome, which here means a mid-rank magistrate
Mid Career
Now the not so young guy becomes legatus legionis and afterwards a proconsul of a minor province. Now he is called a senator, if no censor disagrees.
Second step is a legatus pro praetore commanding more than 1 legion followed by proconsul of a major province
Finally he could return to Rome or become a legatus pro consule, who commands an army of 4 or more legions
High Career
Back in Rome the senator, who should now about 50 years old, if not died in the meantime, could become a member of the general staff or apply for the major offices in Rome like Praetor and Consul.
A legatus pro consule, who commanded the big armies, becomes member of the general staff automatically after his office as legate. Nevertheless, he can apply for other magistrates like every senator. So a top-job in Rome is guaranteed for all generals leading the big armies.
Such a new cursus honorum needs obviously a change in the constitution. This career in a fully different order leads to a situation, where it makes a lot of sense for every general, to lay down his gladius, come back to Rome and go for the real big offices after his military career. And once without an army, he cannot usurp. Finally he could perhaps even become princeps.
If we need a princeps at all. I also have some more detailed ideas for the military reform and would like to propose a reformation of the traditional offices, too. More like the later imperial central organisation but with traditional republican names. And of course, there is still no fully working division of powers in Rome itself, which leads to a reform of jurisdiction. But thats another story.