There as to be an EU3!!!
Not so soon as we've seen EU2 come out, but in good time, and maby with a new engine, it will be needed
But do not extend the time frame. Apart from the military point of view, the concept of the game is covering the periode where Europe made it's impact felt on the rest of the world after waking up from the dark ages, not an history reenactment tool, altough that would be fun.

And anyway people, do you really expect to get a WWI when you start a game in 1400? My games turn out very different from history as it is.
As for improvements...
The culture idea was a good addition for me, but it as to be dynamic than just with events. Cultures is hard to change, but if a goverment decides to assimilate a culture, he as the right to at least try!
Be careful with too much provinces. It's Ok if you are playing Bavaria, but it becomes hell when playing Spain, France or England.
Permanent Terra Icognita that fades depending on the infrastructure level instead of time. That would limit too big exploration of regions that were inacessible too early. Or allowing exploration but limiting colonies.
Diplomacy rating wider than +-200, say +-500, or even +-1000, so you can really piss someone off!