This is sheer fantasy, but it would be great if you had Roman generals that turn rebel and attack Rome.

And if they actually succeed in taking Rome, then all 'Rebel' stacks led by a captain or general of Roman origin suddenly become the Romani faction, while the previous Roman stacks all become Rebel, and the player suddenly ends up controlling the Rebels!

Then, if the 'new' Rebels take Rome back, they become Romans again, and the 'usurping Romans' go back to being Rebels.

Would be funny!