So Legacy of Rome is already out on Gamer's Gate which means I ran out of time on my current project.
It was meant to be my retort to Byzaphiles. Rome is ruled in Rome, not in some Greek hole that's going to be overrun by Turkmen in a century. Plan to export it to EUIII sometime and have fun sacking Rome all over again, maybe as Gotland.
As usual late-game CKII is tedious and boring. Maybe the whole standing army mechanics from Legacy of Rome might make the end game a little less grating but I doubt that.
The key to expanding and having a stable empire is ... Arch Prince-Bishops (duke bishops). Seriously.
They'll always vote for your choice in an election. They rarely plot or claim other lands. They don't desire king titles or other dukedoms. With Free Investiture they gain a +30 relationship boost and you can choose who inherits. If you have one of a foreign culture or bad traits you can just kill him and another will be elected. They don't have a "wrong government" hit. Getting virtues/crusader will push their relationship even higher. Almost always maintain 100 relation.
The negative is that you get less taxes/levies from doges/dukes in theory. However in practice since you'll be taking a pretty massive relation penalty from either dukes/doges, a Prince-Bishop will almost always generate more taxes and levies for you.
The other penalty is not being able to give dynasty members titles without micromanagement but if you're a king/emperor you won't ever be wanting for prestige.