I wasn't thinking all that much when I wrote it. However I still think it could still work.
As far as I know Noble Republics almost always voted in an elective of a handful of the most powerful families in the country.
However as far as I think republics would function in the game, becoming an elective head of the republic wouldn't be the main focus. Sure it might give you a nice prestiage bonus but the real power would come in forming voting blocks, grooming puppets, enacting favourable laws and gaining more land for your dynasty.
I guess it would be a little like the Imperial Politics, but the main goal wouldn't be taking the head postion. Playing as a wealthy land owner who has his tendrils in all political decissions and is able to change laws on a whim, able to easily vote in a puppet who will blindy follow his orders, fighting in the crusade to gain some favour, all while filling his own coffers would be the real goal. I imagine it would be pretty brutal at times with all the amount of intrigue.
Republics would play very differently, which to me is always a good thing. However the main problem would be the handful of republic that existed at this time were heavily focused on trading or were trading republics. Playing Venice without trade, which Crusader Kings II does lack, seems rather odd and almost pointless to me.
Not to mention the mechanics would be rather tricky to iron out, plus you have to get the AI to work rather smart so it feels like your dealing with real humans.
Due to the amount of work required I don't really see republics being playable in any future expansion however.