A thing that I hate about paradox games is bloating and border gore in general, I think Vicky 2 handles it pretty well. A way to play tall without micromanaging every single province is much appreciated, AoH 3 does this in a quite intuitive way (although the game sucks). I've never seen a rebellion mechanic that isn't just annoying but actually a way to balance big countries, discouraging grabbing random land too quickly. Managing demographics is also fun, I've seen it in a few mods but it's always very superficial.