I'm not arguing for Julius Caesar, because I don't want him in again, but to argue against him on the grounds that he "wasn't an emperor" doesn't make sense. Nobody was emperor in Caesar's time, or ever had been. No such position had ever existed, because ever since the kings were overthrown, Rome had been a republic. For centuries the "leader" of Rome was either of a pair of consuls, not emperors. And "dictator", a title Caesar was honored with lifetime possession of, was a real, constitutionally valid office that made the holder the de jure and de facto ruler of Rome--usually temporarily, to deal with a crisis situation. It was Caesar's nephew Octavian who became the first actual emperor as Augustus, and even then he'd have denied being any kind of monarch because Roman sentiments were still strongly anti-monarchist in those days. Augustus preferred to maintain the fiction that Rome was still a republic and he was just a prominent citizen. Later emperors didn't bother with that, but the people eventually adapted and got used to living in a monarchy.
If we insist that only true emperors can rule the Roman civ, then that leaves out a lot of important people who were legitimately powerful leaders of the Roman state. Cincinnatus, Scipio Africanus, Pompey the Great, Cicero, Sulla, and Julius Caesar were all consuls, and some of them dictators, without any of them having ever been "emperor". You can argue against any one of them on any grounds you like, but I think insisting on only allowing emperors to be eligible is short-sighted.
To put it another way, Japan has never been led in-game by anyone who was emperor, but who would try to argue against Tokugawa or Hojo on those grounds? They weren't emperors, but they did rule the country, and they were historically significant. The same can be said for many figures in the first two-thirds or so of Roman history, when there were no emperors.