The closest thing you can ever passably hope for is fallout online and that will be more about teamwork, and less about mingebaging everything within a thousand yards.
Fallout would be the perfect setting for an MMO, I think. provided I can play anyway I want (including being a pvp-whoring ragequit inducing sonofabitch. Because that's what people in the PA will be like. Dicks.
Can I correct you by saying that's what EVERYONE will be like, if you want to play a mmo of fallout go get fallout 2238 it's the closest thing you will ever get to a fallout mmo.