During the Making of Halo 3 documentary (Halo 3 Legendary Edition), they pretty much made it clear they did NOT want to work on anything Halo related for awhile.
Let's look back at the history of their games:
Gnop! - Pong clone
Operation Desert Storm - Tank game
Minotaur: The Labyrinths of Crete - RPG
Pathways Into Darkness - FPS (the first Bungie game I owned, and the first real success for Bungie)
Marathon - FPS (big hit)
Marathon 2: Durandal (bigger hit)
Myth - top-down RTS without resource gathering
Oni - artistic FPS is the best way I can describe it... tanked because of technical issues and delays
Halo 1, 2, and 3 - FPS
Except for Myth, the big successes for Bungie have been their FPS games, so I doubt they will deviate too far from that path. On the other hand, they are a very creative bunch, so who know... maybe they'll surprise us all and create the next great platformer... just kidding.
EDIT: My son believes they might be working on a remake of Marathon.
