I'm fairly disenchanted with sc2 so the warcraft rts family isn't something i'm hungering for at the moment. The first order of business for Blizzard should be putting down WoW in the most humane way possible and focusing their attention (and rediculous bank-roll) on innovation again. I think they'd be best served stepping outside of the box again.
Warcraft3 brough us a very cool combination of rpg and rts. WoW combined some of their great rts lore with a bog-standard mmo (not all that innovative, but at least compelling). Perhaps there is new ground to be broken in some yet-untapped genre and story-telling twists?
I'm thinking a sandbox style 3rd person mmo with a splash of rts resource management/exploitation brought to us in the compelling universe of Starcraft or a Diablo-like world (post-apocalyptic is a bit played-out though). Blizzard seems to be preening WoW in a manner that's avoiding the old traditional grinds so a sandbox platform makes the most sense. How they add their touch of imagination from that point would be interesting indeed.
I was thrilled by wc3 and really liked the dimension of the heros that defined it apart from starcraft. TFT diminished that somewhat, but delivered a much-needed shot of unit-balance. Overall I thought it was a good experience as someone who held the RTS genre dear to his or her heart. Also, wc3 was a great progenitor of the MOBA genre (that I never got much out of, but it is massively popular these days).