Apparently a lot of people think Blizzard's "Project Titan" is StarCraft's evolution into the MMO universe. I would actually love to see this happen... if it is done right. From the sound of things, Blizzard is sounding pretty confident about whatever it is they're working on. I imagine them raising the bar of sorts for the MMO genre as a whole-- in a way future MMOs will continue to imitate them. When I think "Starcraft" and "MMO" combined, I imagine a new kind of MMO gameplay that combines RTS and shooter elements.
Reviewing all the cinematic from Wings of Liberty and Heart of the Swarm, it feels almost like a Starcraft MMO is inevitable. It seems almost certain that after the Protoss have their round about, the next step will almost certainly be an MMO, whereas the Xelnaga are the main antagonist and mutual enemy to everyone, kind of the same way the Scourge/Burning Legion was to Warcraft, secretly working behind the scenes.
I keep thinking, if it is Starcraft, there could be squad-based commands where individual players could possibly control large groups of "pets" or NPC allies to assist them. I'm also thinking they might take a page from the Star Wars MMO and have a sort of space-combat side system-- just hopefully more realistic/immersive and worthwhile... not a cheesy rail-shooter.
Another thing I've been thinking about is how the various races/factions could work. Terrans and Protoss are pretty easy to imagine, but not so much Zerg. Would player-characters of the Zerg be infested Terrans perhaps?
This is not to be confused with the"World of Starcraft" mod. About a month ago some modders came out with their own fan-made StarCraft MMO using Starcraft 2 as its foundation. I have not had the opportunity to try it myself, but I'm assuming it can only do so much that hasn't already been done with MMOs in the past.