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.