Well.. I believe that a Fallout MMO is possible as much as it is impossible. First, I bet it would be difficult to implement the vats mode+all those cool effects online because it would cause plenty of lag. On the other hand, as seen in other MMOs it would be very possible to create a huge open world and populate it with hundreds of quests, enemies and monsters. But then you'd have to give people something to do... and what would they do aside from killing npc monsters? Suppose you give players the ability to join rival factions.. someone could be BoS, others would be raiders or enclave... But wouldn't that be kinda silly without a clever story that would make such things make sense? I think it would be, because there is not much of a story behind the enigmatic enclave, the honorable BoS and the chaotic raiders.... And I cant see why there should be. They are fine the way they are.
A Fallout MMO sounds good in "theory" but probably not such a good idea overall. I'll tell you this much. I'd rather Fallout stay the way it is, then to have it butchered by someone who will make an MMO that will suck. I'd rather Bethesda make the next Fallout, and make that one more like Fallout Tactics with an ability to ride around in vehicles like buggies, tanks, etc. Maybe even one or two helicopters? To spice things up a bit, how about a coop mode where two or maybe even up to three people can go through the campaign together? This time however, I'd love some hostile cyborgs, and more diversity in enemies overall. I'd say it's good single player games we are lacking in lately. There's plenty of MMO's as is.