The rest of the Hive pantheon, or the Kell of Kings. I really want to see more done with the House of Kings. Also the Cabal Emperor, and I imagine there are probably one or more powerful Vex minds controlling the rest, so maybe seeing something like that could be cool. And, of course, eventually we'll probably fight the Darkness directly, which could manifest itself any number of ways. Another less likely possibility is maybe we fight the last Ahamkaras or something like that