Requiem

At the end of cutscene for the Legendary ending of Halo 3, The section of the UNSC Frigate Forward Unto Dawn containing Master Chief and Cortana, is seen drifting towards an unknown planet, when light is shined across the surface of the planet there is a large Marathon symbol on the surface.

Speculation
There is speculation on what this unknown Legendary Planet could be. Some people believe it is Onyx, although its destruction in Ghosts of Onyx make this theory highly unlikely.

The second speculation of this planet is the fact that it could be Tau Ceti IV and that the cyborg portrayed in the video game Marathon is actually a future Master Chief. This is a highly popular theory, as well as following up with all Halo canon to make this theory plausible in the Halo Universe. No one (but Bungie) is entirely sure on what that planet is.