La la la la I can't hear you la la la....
Bah, not working.
Do we really need another addiction? Isn't WoW enough?
Yes, it could be cool. It's an awesome setting, and has the potential to be an awesome gameplay experience, potentially even moreso than WoW, IMO.
Terran and Protoss, I can see. How to play the Zerg? Would you be a Cerebrate, and have 'pet' zerg species you'd call on for attacks/defense? I wouldn't think playing an individual Zerg 'unit' would work, nor do I really think you could play as a swarm of, say, zerglings.
Yeah, the implementation of the zerg as a playable race would be a huge task if they wanted to remain true to the Starcraft formula (3xPvP).
Unlike WC, where really have one basic unit (humanoid) and everything radiates out from that, like armor and weapons, in SC, you would have to have completely different loot for each race. It might follow a similar form, but could not be the same (unless you, for example, wanted the protoss and zerg to use machine guns, which would be silly).
I would think the best bet, even though many would hate it, would just be to allow one faction, Terran, and go from there. It would remove the SC PvP dynamic unforunately, but I think it would allow for a stronger game in the end.