Mechanics you would like to see changed

I would change the religious ideas.
I'd like to see two different sets, one focusing on converting the infidels (boosting missionary strength and such), the other focusing on the tolerance of heatens and heretics (increased tolerance and religious unity).
They would work like the aristocratic/plutocratic ideas, so that it would be impossible to pick both.
Right now there's just no point in being tolerant; with two alternative sets we could prevent Greece from turning sunni in every single game.

I think increasing coring would be dire. Already it takes too long to just swan up and kill everyone, claim the land and make it yours. I think in 10 years anyone could change the admin of a place, maybe add a "Recently cored" modify to the province that lowers tax by a little but think about it like this. You land grab a neighbours county, offer the land to your nobles and some merchants for free, then because of this settlement you have a core on it. Just like NI guys.

I'd like to see merchants changed. The only things merchants should do are let you collect from trade outside your home node and generate trade power from light ships in nodes where you don't own land. You should always be able to chose how you wish to steer as long as you have trade power in a node

1. War Exhaustion. It is too easy to get rid of it; obviously balanced for MP, left for dead in SP.

2. Rebellions. Non existent unless scripted to happen. AI gets -3 RR on top of it.

3. Coalitions. Nations should not be able to join coalitions against countries with lower base tax than they have.

Rewrite every single event in the game into a chain, which is then modified by the decisions you make in prior events.

Bring back at least some of EUIII's 'things happening over time' - particularly stability slowly building up. The way stability currently works makes no sense whatsoever. Stability just does not work like that. The EUIII method was dramatically better. I also think the coring system in EUIII was superior (and the technology system, but let's not get into that. My perfect EU game would have all the sliders from III combined with some of the changes and the graphics of IV).

If I could only change one thing, it would be this:

Instead of every unit in the world being telepathically notified that a battle is going on and being able to march from Plotsk to Normandy while the battle rages, I would require a delay before a movement order could be acted on, with delay related to distance from capitol and movement rating of general.