my theory is that the legendary planet is the forerunner homeworld. ok it sounds lame but there is a lot of evidence. first, the symbols show that the planet is no doubt forerunner. next, in one of the halo 3 terminals on legendary, the AI Mendicant Bias (which protected and assisted the chief on the ark after he is relieved from the Gravemind) said directly to the chief"Atonement.
And so here at the end of my life, I do once again betray a former master. The path ahead is fraught with peril. But I will do all I can to keep it stable - keep you safe. I'm not so foolish to think this will absolve me of my sins. One life hardly balances billions.
But I would have my masters know that I have changed.
And you shall be my example." - Mendicant Bias
This shows he probably had something to do with where the chief ended up and that he sent the chief somewhere to prove to his "masters" he has changed. Also, in the halo legends episode "origins" the Unidentified Forerunner Planet, a Forerunner colony, is indeed the Legendary Planet.you see what looks like the Forward Unto Dawn drifting towards the planet, and Installation 00 orbiting a rocky, brown moon, which in turn is orbiting the planet in question
some other of my theories is that it is the planet Reach, becasue the last chapter of the game is named "Full Circle, and Halo one starts off with the chief leaving reach and climbing out of a cryo pod on a frigate. Also, It is possible that the covenent inscribed forerunner symbols onto the planet when they glassed it. Also, there is a formation on it that looks exactly like the lake on reach that must have dried up after it was glassed. this theory is highly unlikely tho.
Another unlikely theory is that it is the shield world onyx. It would make sense because it is forerunner and it would ultimately set up an awesome storyline for Halo 4 with the Chief reuniting with Mendez and Dr Hasley and some fellow SPARTAN IIs and SPARTAN IIIs
Thanks for reading!