More seriously, I'd love an EU4 that had simplified versions of:
Victoria II's pop tracking and market system (but regional markets instead of one global one)
Crusader Kings II's character system for national and local leaders and dynasties, which could also be used for...
Magna Mundi's faction system, plus whatever other good bits that should be salvaged
Also, a better way to represent armies, with more of the battles actually visible on the map, and a more dynamic map that shows the changing seasons and frozen ports and rivers and the mini ice age and volcanic eruptions and all that good stuff.
Oh, and dynamic provinces!
I'm not going to hold my breath