If the game is completed on the Legendary difficulty, the section of Forward Unto Dawn that Cortana and the Chief are aboard is seen drifting towards an artificial looking planet after an unknown amount of time since he put himself in cyro. Some assume this to be Onyx, but this is impossible, as the Milky Way can be seen in the background, meaning the planet is still beyond the rim.
The ending seems to imply that Bungie is in the works on a sequel to Halo 3, either as a video game, possibly Peter Jackson's Halo project, or the third novel from Tor in Bungie and Tor's three book contract.
This ending also draws an interesting parallel with the opening scene of Marathon, leading some to jump to conclusions and say that Halo 3 is in fact a direct prequel to Marathon. However, this is false since Bungie has on several occasions stated that Marathon and Halo do not occur in the same universe.