[Syrus15]

There won't be. There could be spin offs, but no sequel to Halo 3. There's proof as, the day after the E3 2005 Announcement trailer was shown, they said on the official website (Bungie.net) that Halo 3 is the last game in this story arc. It's proven by Halo Wars being a different arc, as it's a RTS based on the events before Halo: Combat Evolved.

Also, the back of the standard edition of Halo 3 says "conclusion to the trilogy." And since MS has two Halo games in the making (Wars and Chronicles) it has more proof that there won't be a Halo 4, but a fourth Halo game. So no Halo 3 sequel, but a new Halo game all together. Microsoft isn't throwing away a good franchise.

No more proof needed. Besides, Bungie can't make Halo games anymore without Microsoft's permission, since Microsoft owns it. But who wants more Halo anyways? Every game the multiplayer maps get worse, and it'll be nice to see a new game come from them for the 360.