Starcraft MMO would be difficult, but not impossible. All it would take is the Starcraft 2 team deciding what game to work with after Starcraft 2. Read the interview at VideoGamers.
The StarCraft universe is really rich, and it’s got a lot of cool stuff in it that could be leveraged for any number of different genres. A big factor would be what the development teams want to work on. When the StarCraft II team is done with StarCraft II, a factor is what they want to work on next, because the development teams plays a big role in deciding what they’re going to do. So if there was a development team that was really passionate about leveraging the StarCraft universe for a different genre then that’s something we would seriously consider.