Re: So what concepts are you tempted to play on the MMORPG anyway?
I have a concept that I've had various ideas on how to do called The Uncatchable Man. Speed, agility, cleverness, and an uncanny ability to slip out of Entangles. (Handcuffs? Don't even think about it.) Such a character would be relatively little use to a conventional super-team. He'd shine in the sort of solo play where he defines his own objectives -- acting directly against criminals on thjeir own terms rather than reacting to crises.
It would all depend on how much initiative the MMO genre really permits. The two fantasy MMORPGs I've tried have the sort of railroading that would make most live GMs cower in shame.