I wouldn't rule out an MMORPG to be honest since I suspect "branching narrative" and more immersive storylines are going to be what the next generation is about looking at things like both "World Of Warcraft" and what's been shown of the "Knight Of The Old Republic" MMORPG, which included the abillity to influance missions by dialog before they even started (or so it seemed from a certain sequence they showed with a Sith Warrior and Bounty Hunter teaming up in one of their videos).
With things like "phasing" within bigger instances like you see in WoW, where people can co-exist more or less on the same map, but one area might be radically differant to another player depending on whether they have triggered events like "The Wrath Gate" or whatever, there is an increasing amount of potential for characters to have a profound effect on the world, or at least as they see it.
Of course things can be quite spotty with characters trying to work together and "phasing out" to each other within a zone due to the differances... but it's a new technology and being worked on right now.
So I'd imagine if Bungie was sitting there and looking at WoW, and looking at Old Republic Online, and wanted to create an MMORPG to compete at the next generation, they would need exactly the kinds of people mentioned above.
I think that what is going to make the "next gen" MMOs "next gen" is going to be to use a tweaked version of what Blizzard did with "The Lich King" but introduce it as an integral part of the game from day #1.