It all comes down, essentially, to the statement by Microsoft that the next Halo title won't star Master Chief. "Lateral thinking", as RPS puts it, can easily see the possibility of yourself stepping into the 'title' role in an interaction with the franchise. Certainly the groundwork is there - we know that Bungie has some lore tucked away behind the scenes. They know lots about that world we haven't seen in the games yet, and odds are they'd love to get it out somehow. Halo 3, by the same token, has a bunch of MMO-like elements already embedded. Much like Call of Duty 4, your participation in multiplayer matches impacts a persistent 'character'. The site has a few links for you to follow (including one to an interesting Microsoft research paper), and some videos as well.
All of this is just speculation, just the same. What do you think? Are we likely to see a brand new Bungie-helmed MMOFPS sometime in the future?