My Vicky 3 wishlist:
They need to correctly model the different political systems. Vicky 2 was basically the British system for every constitutional monarchy. the political realities didn't work for Prussia/Germany at all, for example.
The same with religion.
First of all, the clergy being responsible for education and science is stupid beyond belief.
Second, for Germany, again, the political realities of the Protestant (Nothern Germany/Prussia) and Catholic (Southern Germany) strife within the Reich was completely absent. The Zentrumspartei was a catholic conservative party and was always thrown together with all other conservative and reactionary parties in votes in Germany. While the reality was, that the Zentrumspartei was basically a state enemy of the Kaiser and Chancellor.
Research: I would like a research system like the game "Pride of Nations". You would passively research new systems by things you do and finance, which is more based on reality then the head of state saying "Hey eggheads, invent me Nitroglycerin!"