Or in other words, Batman Beyond and Iron Man versus Halos 1 through 4.
Right before Chief wakes up from his cryopod in Halo, one of two situations occurs: He is either replaced by Terry McGinnis in his futuristic Batsuit, or he is replaced by Tony Stark wearing his classic red-and-gold armor.
Both receive knowledge of UNSC battle tactics, chain of command, etc, are capable of carrying Cortana with them, and are forced to react to the immediate Covenant threat. As each new alien enemy approaches, they will instantly gain the same knowledge of them as the Master Chief and Cortana.
The entire universe has been placed under a genjutsu that makes everyone perceive Terry or Tony as "the last hope for humanity" or in the Covenant's case, as "the demon".
The only physical objects Terry and Stark keep from their universes is the powered armor on their backs. Can they make it to Halo 4?