You could make baronies in republics playable, so even if you lose the Doge title, you still have your barony. With something like the Senate from EU:Rome, you could make it quite interesting.Even if you continue playing when out of power, to alter things so that the player character has some sort of power to act when they're not the ruler would be a pretty stupendous overhaul of the game.
Expansion of republics is more of a trouble IMO, maybe if said baronies could not hold any other titles.