Requiem

In the Legendary ending of Halo 3, the rear section of the UNSC Frigate Forward Unto Dawn containing Master Chief and Cortana, is seen drifting towards an unknown planet. Seconds later, a light shined across the surface of the planet. If a user freezes the cutscene, they can see that there is a large Marathon symbol on the surface of the planet. The symbol has lead to much speculation to whether it is Tau Ceti IV or just another in a long line of marathon references.

Background
There are many speculations on what this unknown planet might be. Some people believe it is Onyx, although it does not seem to match the description of the mechanical planet found in Ghosts of Onyx, making this somewhat unlikely. Also the fact that Onyx was found out to be millions of Sentinels put together. The assumption of the is planet being a Forerunner Shield World, could be possible. The Ark portal would have to have a failsafe location to transport to. Some also believe that the terminals in the game are a reference to the unknown planet. However, it is clearly inhabited by some advanced species, or was at one time, and by the Forerunner symbols, they are the natural alternative of inhabitors, at least in the past.

Marathon
It was postulated early on that this planet was Tau Ceti IV and that the cyborg portrayed in the video game Marathon is actually the future Master Chief. This theory coincided well with the concept of the "Eternal Hero" Bungie is famous for, however the resemblance seems to be only a nod to fans. Bungie has confirmed the Halo series and Marathon Series do not take place in the same universes.

Sangheilios
Another theory includes that the mysterious planet is actually the Elite homeworld of Sangheilios. Although the image of the homeworld in the Beastrium do not match the picture, many Halo fans still believe that this planet may be Sanghelios. Another fact leading to another answer than it being Sangheilios, is by the time John-117 and Cortana arrives, The Arbiter and Rtas Vadum has probably already arrived, as by the end of Halo 3, they headed home, and they most likely traveled by Slipspace due to the distance from Earth. This would leave no openings for an entertaining storyline in a game sequel, thus render this alternative most unlikely.

Forerunner Colony
Yet another theory is that it could be a Forerunner Colony or homeworld. Throughout the Halo franchise there have a few mentions of the Forerunner colonies prior to the discovery, and subsequent war with the Flood, notably in the Halo 3 terminals. This theory provides an alternative to the "Shield World-theory", as well as explaining the planetoid, the Forerunner symbols, and providing an opening for sequels. Another point supporting the theory is that there appears to be oceans and cities on the surface, similar to those seen numerous places on the Ark, excluding the theory of Sangheilios, and most likely the Shield World-thory as well,

"Rainbow World"
One final theory deals with the legendary planet being similar to a Star Control II "Rainbow World." - This would explain the references to the Precursors in terminals and other supplemental material throughout the game, and explain why Didact wished to "Follow in their footsteps," as the Precursors made the Rainbow Worlds of Star Control to point the direction in which they migrated to.

Location
Even the location of the planet is a mystery. Some believe that it is located relatively close to the Ark, basing this theory off of the nebulous clouds seen around and behind the Ark and the portal also being seen multiple times in the ending sequence. Others think that the Frigate managed to get halfway through the portal, and was dumped out at a random destination, or even a planned failsafe area.

Although its purpose and identity are obscure, the planet is an elegant story device to keep the series open to future game titles or novels, and leaves the opportunity for the Master Chief and Cortana to continue their adventures well into the future.

Trivia

 * One of the A.I's in Marathon named Tycho had crashed into one of the Lh'owon moons, as stated by the U.E.S.C. Marathon A.I. Durandal. The back of Forward Unto Dawn crashing on the planet would be another reference to Marathon.