Well, in Luthier's words, it has to be:
A. Cool

B. Major
C. Able to make its competitors squirm
So here's my 3:
1. First person scramble along with all those 'shelf loads of crew animations' that Luthier mentioned a while ago.
2. Dynamic weather (though we all thought we'd got this when we bought CLOD in the first place)
3. Playable AA ground crew or, at a push, playable air-sea rescue for downed crewmen.
My money's probably on dynamic weather, though thats been talked of a lot as a feature that was not finished and hit fps badly. Luthier's comment makes it sound totally new and unique, i.e. Not something previously talked of as planned or originally intended, but its hard to imagine theyve had the time to implement something completely new AND major, I hope he meant it!