About the only factions I'd are Serbia, Lithuania, and some Muslim Perisan faction (I don't know enough about them to make a judgement on which).

Why Serbia?
Well of any possible Bulkan factions they could made different enough from Former Romania or Hungary to be intesting. And they were never more than a protectorate of Former Romania (unlike Bulgaria). Plus it's always good to have more orthodox factions in the game.

Why Lithuania?
Not having the eastern end of the map nothing but rebel provinces would be nice. Lithuania was a major regional power in the north east. WArring against the Russians, the Poles, and the Teutonic knights. Plus until the mid game they'd be pagan. So the idea of being able to convert from the old gods to eastern or western christianity or even Islam could be interesting.

Adding a Muslim Persian faction is good thing because we need more Muslim factions. The only one I know of is the Khwarizmian Shah's though. But they might be off the map.