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.