Admiral Cole supposed died at the Battle of Psi Serpentis. But, there was a chance that he escaped via a last-second slipspace jump. How would you feel if he showed up in Halo 5 or 6?
I think, if it worked well with the story, it'd be a fantastic nod to the classic Halo lore, and really help tie in the old with the new. Only if they made it work, though. Anything half-assed would just be a disappointment.
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I can actually see that happening (maybe not with Cole leading). With the temporary destruction of the Covenant rebels could easily get old covy, unsc, and forerunner tech (from covy stockpiles before and during the war) on the black market. So rebel groups might have a chance. I just hope we don't end up with a Stalin or Castro running things. It would have to be someone who could really set things straight and get an effective government up and running.