Caesar Imperator, Hero of the Republic: Rough Outline
I realize I was somewhat dismissive of this idea earlier, but I've been thinking about it for a while and I keep running into the issue that most outcomes of Caesar being killed in Gaul lead to a less compelling century of history afterwards.
However, I think I've stumbled into the beginnings of an interesting deviation. Specifically, I've been trying to think of a way to have Caesar die before he can cross the Rubicon while preserving the myth of Caesar. And I think I'M almost onto something.
The first point of divergence in this proposition would have to be during Caesar's time as First Consul. Specifically, I propose that Caesar, being a great admirer of the great lawyer and orator Marcus Tullius Cicero, chose to invite him, not to join with the Triumvirate like in OTL, but rather as a ''second pair of eyes and a sound mind'' to look over his policies throughout the course of his Consulship.
Now, I acknowledge this would be very unusual on Cicero's part but, I do think that, while something of a stretch, it would be within the realms of possibility for Cicero to accept such an offer with the reasoning that, in doing so, he could at least mitigate some of the ills he foresaw in Caesar's designs. Mind you, this would have to be at the very beginning of the latter's Consulship, as no force on earth could make Cicero cooperate with Caesar after his very public circumvention of the law whilst in office.
The reason I suggest this is that doing so would afford Cicero a great deal of insight into the reasoning behind Caesar's policies and give him some first hand experience of the Senate putting it's own interests before those of Rome. With him having been such a strong idealist, I imagine such an experience might well trouble the man greatly; perhaps even to the point of reconsidering some of his ideas which he had thought unassailable?
You see, the interesting thing about Caesar is that, unlike most politicians and/or statesmen driven primarily by ambition, he seemed that have possessed a strange sort of personal honour. He steadfastly refused to conceal either the existence or intent of his (public) actions and policies, he always kept his word once he had given it, he never betrayed or failed to come to the aid of an ally (often to his detriment) and, most importantly, made absolutely certain that, in addition to aiding his own cause, his policies served the public interest and actually improved the situation for Rome and its citizens.
Now, the above is, I fully admit, the result of a favorable predisposition on my part but its is, at the very least, not an untruth. I imagine you can see where I'm going with this at this point, but I'll connect the dots for the sake of completion. The notion I had was that Cicero's years of working with Caesar on the redrafting of half the Roman legal system might have given him a measure of appreciation for the man.
Once again, Cicero was a staunch idealist while Caesar was the essence of pragmatism, so I don't imagine they'd actually have agreed on all that many issues. Instead, I propose that Cicero might have come to admire Caesar's goals (his every move while Consul was directed at safeguarding the welfare of Rome and, most importantly, its people) while still disproving of his motives. However, this new found respect for the man, while not bringing him to the point of supporting Caesar's actions, would possibly suffice to convince him not to publicly condemn them and perhaps, if those presenting the arguments were to be particularly tasteless in their accusations, defend them to some degree.
While this might happen on a number of occasions, I have something specific in mind that could serve as a useful catalyst. You see, Cicero was, in spit of his brilliance, a very straightforward man who saw things in an almost completely Manichean manner, and two wrongs don't make a right. As such, when the Senate sees fit to try and sentence Caesar without the latter being present to defend himself, I can see a man such as Cicero being incensed at the Senate casually disregarding Republican institutions at least nominally in order to punish another for doing so. In fact, in this sort of context, I can see such a scenario convincing the man that the Senate, in its current form, did not have the best interests of Rome at heart.
After such a change of heart, I can conceive of Cicero (who was held in very high regard by the Optimates) persuading those Senators who were sincere in there motives (such as Cato the Younger) to, at the very least, allow Caesar to account for his actions and defend himself in person before any sanctions were applied. Mind you, there would be no suggestion of simply letting the matter drop, merely that judging the man with a kangaroo court in his absence for excessively bending the law while in office would be hypocritical in the extreme.
Any, that's the set-up in Rome taken are of: no public accusations against Caesar are made in his absence and the Senate, as a result, does not put him in a position where he has no choice but to, if you'll forgive the phrase'' cross the Rubicon. While this would make Caesar's position less perilous than in OTL, it would still leave him with the necessity of accomplishing something extraordinary as political capital for his defense (Cicero having sent him a letter detailing the situation out of respect for his former colleague). This would mean that he would still need to conquer Gaul.
At this point, I propose that the war in Gaul proceed as in OTL (it is instrumental to the myth of Caesar), with one minor exception: on the ast leg of his campaign, while he was preparing for his (half-hearted) assault on Britain, he meets with a son of the chieftain of the Cornovii (based in modern day Cornwall) an through speaking with the young man, learns of the substantial tin deposits located in the region. Now, before learning of the presence of massive amounts of tin being present in Britain, it had been regarded as largely worthless (which is why Caesar let the matter drop OTL). However, substantial tin deposits are quite rare, wars are fought over them to this day, and the presence of such a thing in Britain change things completely.
As such, due to his legal status being ever so slightly less shaky than in OTL, Caesar (never being averse to risk), pens a missive to the Senate explaining his discovery, along with a request for a short (6 months or so) extension of his pro-consulship to allow him to capitalize on his momentum and finish off his campaign without any loose ends. Now, seeing as Caesar had already conquered Gaul at this point and that tin (and by extension bronze) really was that valuable, I can see the Senate indulging such and extraordinary request, the Optimates because it would clearly add to Rome's strength (Caesar had, if nothing else, proven beyond a shadow of a doubt his capabilities as a general) and the Trimuvirate's (or at least, what was left of it) supporters because, in Rome, you backed your patron in politics and that was that.
With this extraordinary extension being granted, Caesar would have the time to plan his invasion of Britain properly and would have no need for a rough naval assault. This extra allotment of time would allow him to enlist the assistance of locals familiar with the difficulties involved in crossing the Channel and thus allow he and his men to adequately prepare for the crossing. As a result, he successfully lands at the head of a substantial force and, given the he is Caesar is now fighting land battles, successfully subjugates and occupies Britain according to his usual modus operandi (with his good friend and ally, the new chieftain of the Cornovii (let's call him Artorius for the cheek of it) being appointed as governor of the new province of Brittania.
Having succeeded in his conquests, Caesar returns to Rome armed and ready to face the Senate (he, unlike Cicero, rightly does not believe he had any chance of seeing a fair trial). However, Caesar was hardly a young man at this point and, on the way bay to Rome, tragedy strikes. One morning, his generals note that he is uncharacteristically late to rise and decide to investigate. They find him lying in his tent, quite dead from having suffered a stroke during the night.
With this sequence of events, Caesar would quickly turn into a semi-mythic, larger than life figure. You see, while the Senate would never have backed down while Caesar was alive, it would have been in poor taste at best and political self-mutilation to publicly condemn a man who conquered the Gauls, the ancient enemies of Rome who had once sacked the Eternal City itself, in an extraordinarily short time and even added the province of Brittania, with it's rich tin deposits, to Rome's sphere of influence. As such, I think it is plausible that, in this scenario, the Senate would let the matter drop and, in a display of comical hypocrisy, proceed to heap honour after honour on the memory of a man of which they had spoken naught but ill for the past several years.
However, no chapter Roman history is incomplete without a good civil war. Thus, I propose that Caesar's generals elect to pick up where Caesar left off. As I find it exceedingly unlikely and very much out of character that he would choose to conceal what he was doing from his generals (or the rest of his men, for that matter). Therefore, given what we know of Marc Antony's temperament, I find it quite plausible that he would find the idea of installing himself as Dictator with his colleagues serving as his lieutenants appealing for it's own sake. And, being that he was quite popular with the rank and file, I suspect he would have relatively little difficulty convincing men who had spent the last couple of years going from victory to victory on a consistent basis that his was a grand and glorious plan. And so, it would not be Caesar, but Marc Antony who would cross the Rubicon.
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This is very rough, and much of it is based on supposition, but I think it's a decent starting point for something interesting. I realize this is probably not what the OP had in mind when he asked what the outcome of Caesar being killed in Gaul would be, but I got stuck on how to make such a situation, in my own words, ''interesting'' and got stuck on the idea.
In any case, I have a half formed idea on where to go from here. Mostly, this would involve a civil war with both sides claiming to be Caesar's heirs and using his name as a battle standard. As you might imagine, Octavian (better known as Augustus), being Caesar's actual legal heir (he had, in fact, been raised by Caesar's sister Julia Caesaris) would not stand idly by while someone else claimed his inheritance.
In any case, I'm sure I got a few things completely wrong, so, thoughts?