Hey, have you played Crackdown? No? Me neither. Not from lack of interest, mind you; I just don't have an Xbox 360. I foolishly made the decision to eat instead of buying one. It sounded great to me, though: an adventurer that allows you to upgrade your attributes through use -- keep using your gun and your marksmanship will improve. Keep tossing things around and your strength will increase. Just like leveling up, only not so numbers-oriented.
I started wishing I could play it on my computer, which led me to the conceit you read before you: a Crackdown MMO would do me nicely. Think of it: play as an agent of, well, The Agency, the law enforcement entity of this world. Or, positing that the enhancement program that fuels the special abilities of Agency officers is stolen and sold on the open market, play as one of the 3 rival crime syndicates instead. PvP is built right in!