I do like this theory. However, if you look carefully at the planet for the 2 seconds it is shown, it has the 3/4 circle pattern with a vertical line on the top, with more designs inside. Im too lazy to go get a pic of it. However, if you compare this with various images of forerunner planets and symbols, they are the same.
What does this mean?
Master Chief is now landing on another forerunner inhabited (or forerunners used to inhabit) planet. Could the marathon planet and universe somehow also be connected to the forerunners? Possibly. But I think that given the gigantic symbol appearing on the middle of the planet, cortana and the chief are crash landing on yet another forerunner planet.
Could this still be the marathon planet, and we later find out it is also a relic of the forerunners? Quite possibly.
Honestly, I think bungie added the legendary ending just to drive us all bat--blam!- crazy coming up with theories and that they never will elaborate on it.
But if the legendary ending does show anything, I presume that it is showing John 117 and Cortana heading towards a forerunner planet, whether we have been there before or not. And who knows? It could be yet another forerunner planet infested by the flood. Or it could be a secret weapon of the forerunners not yet discovered. Or it could be Marathon. Or it could be a dry piece of rock where the master chief starves to death (yeah I know you hate me for saying that). But honestly, i think it's impossible to tell in 15 seconds of cinematics and only a few words of dialogue. SO we just have to be patient and hope bungie makes another kick ass game starring everybody's favorite "green giant".
And if they do make another adventure starring our hero, it probably wont be nearly as good (no offense to bungie). They are going to have to do some amazing things to make a good plot that makes a good game not involving the near-extinction of humanity.