What CK2 sorely misses are proper opponents for players. The game already has all those flashy realm government mechanics, RPG mechanics, intrigue possibilities and all other tools to create amazing gaming experience. Problem is that you don't need any of those because AI is too stupid to be a real threat in any capacity, both internal and external. AI has no capability for plotting into the future and is always predictably reacting to immediate situations making their defeat just a matter of time. So, what's the point of playing if you've already won?
Imagine a CK2 game where AI would actively fight you from the beginning with all the available tools instead of being a range duck. And I'm not talking about being suicidally aggressive like some mods do. Imagine a game where you are not the only agent who is plotting future events. Where people really fight for power, where blobs are constantly rising and torn into parts, where your count-emperor journey is really hard and rewarding because lieges are not idiots who can be overrun with few mercenary bands or assasination spree.
How can PDX achieve this? I don't know, but I know that this would make the game exponentially better.