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'd rather he stay "retired". He definitely deserves the rest. However, if Nylund comes back to write him (and more Halo stuff) I might be okay with it.