I became convinced that Procopius is nothing more than an impostor. Only Zosimos speaks of Julian appointing him as Caesar, contemporary authors aren't mentioning this.
Because Julian never promoted him Caesar. What Julian did do however was allow Procopius to act as a diversion to lure Shapur north while he struck south with the main force.
Julian has to stop before even declaring war on Persia. The Empire has other problems than expanding in the east - you maybe remember the problems of my last timeline: the Empire is overstretched, an ambitious governor in the east can reach almost everything if he uses the wealth and the manpower of Mesopotamia. But this was Trajan, in a time when Rome had to fight on one front only.
And then Julian is shocked, shocked, when the eastern legions proclaim a usurper against him when he heads back west. Julian had the right idea. The eastern legions weren't too fond of him-a short while ago they were marching against this upstart usurper, and suddenly he's their emperor. He needs to win their loyalty. That is done by winning a war against Persia with, if a couple less hiccups are made, is very doable.
Also, he was
never trying to conquer territory.As I said, he had a concrete goal: Place Shapur's brother on the throne of Persia while sacking Ctesiphon, thus making Persia a non threat for a short while and winning the loyalty of the eastern legions.
Now, every border is attacked, and I don't think Rome has the means to occupy Mesopotamia or even Persia
Julian never wanted to do this. .
Even beating the Sassanids will be hard to achieve, due to their superior tactics.
What?
In fact, it would be better to adopt a defensive stance on the east and concentrate on the Rhine and the Danube border, since the nomads are coming and are far more dangerous than the "civilized" Persians.
For Julian, the best scenario for him was winning over the loyalty of the eastern legions. The best way to do that is sack Ctesiphon and return home.
To resolve the problem, I have developed a funny POD: when marching to the eastern border, the army stopped at Carrhae IOTL. Now, imagine Julian having a nightmare in which he his beaten like Crassus was 400 years ago in the same place. As a religious man, Julian will certainly shrink from marching on. He will accept every offer of peace made by the Persians (maybe a symbolic Roman victory to satisfy the army, but nothing more) and then march to the west to deal with the Alemanni and Goths.
And then the legions will view him as even more of a coward, given that the main Sassanian army is up in Armenia. All it takes for Julian to win the war is for his army to not stall at the gates of Ctesiphon. It's not that difficult.
Thus, he survives 363 and can still try to become the second Alexander later on. I'm sure that he can live until at least 395 AD (he would be 64 years old, that was the age in which Trajan died).
He survives 363, but upon returning west to deal with problems there, is met with the news that the eastern legions are in revolt against him and so may not survive 364.
We had this already in Optimus Princeps. It's simply to dangerous to have someone in the east who could become a potential usurper with the ressources of the whole east!!! THough, if an Emperor is imaginative enough, he could easily make sure that his representative in the east is closely supervised by a council of generals and a secret police. Also, give a certain perspective to such an official: one could become consul, after that prefect of the east, and be sent to Rome to hold an important office. If you know that your carrier isn't at its climax, you are much less tempted to use force to stay in your office.
Procopius would almost certainly be loyal. The problem is the eastern legions won't be unless he can prove to them they should.
As I said, Procopius is, in my view, a crook; but if you're right, he could become commander of the East (Magister militum per Orientem), though keep in mind my objections to such a mighty office.
Procopius rebelled against Valens when he was about to get executed. When backed into a corner, it's no surprise he made a gamble, and its equally no surprise that he forged a backstory that made it look like he was legitimate.
Every usurper will go to great lengths to make them look legitimate.
The point is, during Julian's reign, he showed he was willing to rely heavily on Procopius. Given that Julian doesn't have many people he can trust yet, and given that Procopius is a cousin of his, it stands to reason that Procopius is a man Julian will look to to rely on, especially if he doesn't have any kids.
Julian would certainly find a way to secure his succession. But I advocate to wait a bit and than choose the most capable instead of promoting a child from his birth on.
Except, more than most likely, that's not what Julian is going to do. The history of Rome in particular has shown us that when an emperor has a living male relative, one of his male relatives will be his heir. The five good emperors was remarkable because the emperors
didn't, and even then, I believe the Antonines were at least distantly related in some form. Assuming Procopius is alive or his children are, one of them will be Julian's heir.