Sounds like a nice idea actually. Given so many other nations sought to legitimise their administrations through the Roman heritage, it would be great to see an Italian faction doing just that. It wouldn't actually be all that hard I imagine actually. Simply mod the Italian States to make themselves the "Roman Republic" when there is a popular revolution - so change the flag, some new units maybe - Line Infantry become Legionaires of the Line? - and from then on it's up to the player to choose whether the revival succeeds or not
Look to the 19th Century Attempted Roman Republic for inspiration. Though it didn't succeed, the professed goal was a revival attempt of sorts
It's not a historical idea, but it is
realistic plausible
As for the Byzantines, I think it is better to just perhaps keep it as a Greek revolt - does the Istanbul province revolt to become Greek? Never had it have a national revolt yet, so I wouldn't know. But that's just my opinion