You're wrong when you think they'll be eating away wow fans. There's a large line drawn between sci-fi and fantasy. To give an easy example: look at the amount of wow players versus the amount of star wars / eve online / anarchy online / whatever players. Is wow that much better?
Check the release dates:
star wars galaxies: 2003
world of warcraft: 2004
Best proof that sci-fi is for a really small fanbase, while fantasy appeals to a much larger playerbase. Fantasy simply attracts a different kind of players than scifi.
The same can be said for all the warcraft vs starcraft discussions. They're 2 different games in 2 totally different universes. Warcraft 3 won't die when starcraft is released.
Back to the subject of a SC mmo:
I can definitely see it work. It can be revolutionary on many areas:
- Skills & talents will be much less important, the game should be twitch-based. Marines get to aim their guns, zealots get to dodge and do some melee fighting, rather than sitting back and spamming some macro's for healing and damage spells.
- Dynamic world: you, with other players, influence what happens to the universe. New events are constantly added through patches and expansions. Such events only get unlocked through events caused by players. To give an example: when, thanks to players, zerglings start getting too many worlds under their control, a certain "terran" event could be triggered that gives the terran an advantage over the zerg (think: psi emitter). At the same time, if this weapon needs to be transported and gets intercepted by one of raynor's pirates (also a player, ofcourse), raynor's faction would get this weapon instead.
- It would also be the first MMO to give a decent vehicle support. Vehicles would play an important role, especially spacecraft.
- On the story side: the players create the story, blizzard only provides a bunch of events that will respond to these actions.
Or they could also stick to traditional gameplay, which will probably be a failure.