I have always thought that if Lucas had been thinking (if he were capable of thinking) he would have done something like that for the Star Wars prequels. I mean, you have the huge territory controlled by a corrupt and ineffective Senate, the gradual rise of generals to such extraordinary power that the system can no longer contain them -- then an autocratic movement which portrays itself as a return to old-fashioned, uncorrupt ways, but in fact destroys everything meaningful about the old order... It could totally have worked!

So that's my structure: an oligarchic system on its last legs, warlords contending for power, Caesar forced to start a civil war to defend his own position... I'd probably make the patronage system stronger, so that soldiers were more dependent on their generals. And maybe, since this is "Rome" and not actual history, focus not on the military stuff but on Atia and Octavia as powerbrokers in their own right -- moving it into outer space, I could get rid of the Roman gender expectations. So in this story, I think it would be Atia and Octavia who actually brought about the revolution.