I'd love to see a competent PvE-oriented RTS with the MMO-lite treatment: co-op missions, entire campaigns, customizable armies, unlockable units, mission rewards, purchasable (with in-game currency) items, bigass raids, the whole shebang.

SC2 co-op mode took a big step in that direction, and it was well received and has become fairly popular...but a game built around that as its primary locus could be so much more.

I know many more traditional core gamers would practically vomit at this idea, but I think there's room for plenty of different styles under the RTS umbrella. And bringing in tons of money with one style might well benefit the others.