If there were a SC MMO there'd be some radical changes to the classes, out of necessity. A well-played Pally is still pretty formidable, but they're nothing next to Arthas and Uther. By the same token, Warriors are far, far more powerful than Footmen ever were (Or Knights, for that matter). So by the same token here, there would be massive changes of skills and abilities to balance things out.

It would be great to see three diverse races without identical classes, though.

And yes, I would be Protoss. They are sexy.