The franchise is certainly able to support an MMO, but I'm just not sure if I'd actually play it. What little experience I've had with the genre has left me scratching my head and wondering why it's so damned popular. It's an overly complex RPG with a shallow (and mostly text based) story with the kind of social interaction that only really works if you join a large clan and make it a second job or something. It' like the futon of gaming (neither one thing nor t'other.)
Not sure if I'd want to see the franchise go the FPS route either. Besides Valve made games, FPSs aren't exactly known for their well written narratives. From what I gather they're increasingly just a flimsy excuse to sell gamers a slightly updated version of the same multiplayer game they're already playing.
I think the preferable option would be to stick with the same essential action-RPG format and just start a new story with a different character.