He had a great ending. I know he was a fan favorite character but even good things need to come to an end. If Cayde ever came back it wouldn't be the Cayde you knew. Honestly based off lore we do have, if Cayde came back he'd probably be evil. He's a Shadow of Calus in the Opulent Scholar Bond lore text when you go to the database of exo memories your ghost tells you that Cayde's backup is there but we need to let him rest cause it's what he would have wanted plus he wouldn't be the same. Granted, you can't take Calus' AO3 account seriously, but my point is he wont come back... At least not him as in the Cayde we knew.