I'm all for an SC based MMO, I mean the hero units in the SC universe were effectively regular units that were just that little more bad-ass. And I imagine, like all the decals in the artwork, it'd be fairly easy to conceptualize every marine being a little different in their weapon load-outs and whatnot.
Also I'd love a Pokemon MMO but it doesn't really work with the way the games were made, where there is effectively 1 protagonist and that protagonist catches a bunch of 1-of-a-kind legendary Pokemon, doesn't really work with a few hundred thousand players. Also the "world" is huge and the battles in the game are all instanced so it'd have a pretty different feel from other MMOs. While that isn't bad, new isn't always adopted and treated so kindly in the game industry.
...Also imagine having to add an entire world every time Nintendo launched another title that takes place elsewhere. And all the people QQing that they can't use their hundreds of Pokemon they already have in the games.