The most popular theory is that Bungie will announce a new third-person shooter set in the Halo universe where players take on the role of ODSTs (Orbital Drop-Shock Troopers) during a Covenant attack on Earth. Two possible storylines include a game based on the events between Halo 2 and Halo 3, also seen in the Neill Blomkamp short film Halo Landfall, or showcasing the initial attack on Earth. Both theories ultimately end the game with an epic battle reminiscent of the famous Halo 3 diorama. According to a reliable source, GameSpot says a tactical shooter is likely and considering former Ghost Recon Advanced Warfighter creative director Christian Allen is the lead of Bungie's next project it makes sense.
Another theory after the jump.
Another theory based on the recent 'Superintendent' marketing blitz is less sexy.
Bungie.Net forum goers are speculating the 'Keep It Clean' slogan actually hints at a Halo 3 rank reset -- something Halo 2 players are familiar with. The theory comes after a Bungie.net moderator supposedly posted the phrase '0EEP IT CLEAN' in a thread about a possible rank reset. Other nods to the possibility include a deleted wallpaper (seen above) that asks people to 'remain calm,' which some speculate is a nod to the passionate response a reset will likely trigger.
We don't know for sure but we do know that we'll find out shortly if Bungie has anything to say on the matter.
[Thanks to drew s. for the rank reset tip]