The first thing to address is the why of a post-apocalyptic MMO. The fantasy genre offers us an escape into a magical world. Superheroes allow for all sorts of wish fulfillment. So what does a big, giant wasteland offer up? The human endeavor and the chance to help or impede it. Fallout is a world where corrupt corporations, mislead governments, slavers, raiders, mutants and worse all threaten humankind. Wasteland Earth is practically a siren song for challenging heroics, and an easy temptation for cruel villainy.
We often hear concerns voiced for the surroundings a wasteland would offer up. Cases are made against a vast MMO filled with grays and browns of demolished civilization. For some reason, even though Fallout is founded on a 1950s that lasted until 2077, was blown up and is now full of giant mutated creatures, zombie-like Ghouls and other crazy larger-than-life ideas, it can't possibly be imagined to have diverse -- even beautiful -- locales. Let's not forget that Fallout 3 is set 200 years after the bombs have dropped. Nature is no slouch, it does not rest on its laurels. Give it a couple hundred years and nature will reclaim much of the world of itself, actually. The artistic thing to do would be to take creative liberties, paint important places with greens and other colors. Then, rely on the world to offer you awe-inspiring imagery. Lots of people are familiar with Times Square or the Seattle skyline -- just imagine the powerfulness of that imagery blown up and worn down.
Then we come to the subject of combat. The V.A.T.S. combat proves that RPGs can deal with guns in an interesting way. There's the Tabula Rasa route, where gun-play is handled like an FPS but calculated like an RPG -- although that method seems to please less players than it satisfies. The only reason Fallout 3's combat is so enjoyable is because of the pause action of its combat. Being able to target specific body parts is a huge boon to the game, because it adds a nice tactical layer on top of what would otherwise be a run-n-gun experience determined by dice rolls. So how to deal with the problem of players pausing the game indefinitely? It's simple, really, just add a timer. Allow players to have limited turn-based moments, but keep a timer involved so that the pause can't be abused in PvP. It'd even be worth testing to see if having a timer only in PvP situations worked just as well.
An MMO that offers players a chance to truly experience a world in dire need of help, that's what a Fallout MMO should be at its core. And it needs to be Fallout because of the spirit, lore and ferocity of the world that's been created. Although combat and exploration would most certainly play highly important roles too. There really isn't any reason for a Fallout MMO to work and work quite well. Worryingly, the nearly lifeless husk that is Interplay currently owns the rights to such a thing. So even if it does come to fruition, we may not really care to play it. I hope that never comes to pass, though. I'd rather not ever have a Fallout MMO than see a mediocre or even bad one released.