Halo has a unique position in the world. It was a superior FPS that was kicked off it's throne that it had owned for years, and the developers response for the future of Halo strangely wasn't just "make a better FPS."
It seems that Halo, more than any other game, has been branching out trying to find another niche instead of purely keeping to what it does best. Could MS be trying to strike gold like Blizzard did when they switched Warcraft from RTS to MMO? Probably, but I don't think the transition from FPS to anything could go too well. Their RTS was lackluster, the MMO doesn't look too promising so far, and I'm not sure just how far they're planning to go.
Though transition to RPG might work, if it's done well, such as how Borderlands or Fallout tried to merge the two together. The FPS/RPG genre is still a fledgling, and hasn't found it's perfect slot yet. The definitive FPS/RPG killer app just hasn't come yet, and maybe they can either put one out, or make a stepping stone.
Just a note: While Fallout has been a phenomenal success, it's more RPG than FPS, also, the purely single player aspect takes some away from the FPS' most popular pastime: multiplayer. Borderlands is great, but it gets rather draining over time, because, again, no competitive multiplayer.