Too add my comment...
I would love to see an expansion of the single player game, specifically a campaign based option (and if you included a strategic map a la Total War I will love you forever!) where the excellent unit statistics (especially unit kills) can be built upon and improved.
I argue this as the ability to become attached to your units is a great asset, and every commander has his favourites after all...
As for what I would like to see improved - the vehicle 'sutoroute' system was always a bit clumsy, and the information about units could be made a bit more 'Layman' friendly - especially about things like different weapons on tanks etc (I still don't know the difference between half of them in CM3!)
Probably won't happen, but at least I've said it!