Cayde-7 can theoretically happen, but not in the way you think.
Cayde-6, our Cayde, is dead. Gone. A memory. We could technically get a "different" Cayde via the Deep Stone Crypt if his old conscience is still there; but, even if it is, it'll most likely be the conscience of Cayde-1, someone who does not and will never have the memories (or powers) of our Cayde, even if we reset him up to 7.