As for me, I remember way back earlier in Reach forums when people were discussing the time travel theory: A theory that linked the Halo 3 legendary ending to him possibly traveling back in to planet reach during the fall.
[Edited on 10.05.2012 8:25 PM PDT]
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The theory where Master Chief went threw the portal and wounded up in the Marathon Universe, where he is the main Protagonist. That was an insane theory!