One last time
Speculation on the Leviathan raid:
There's a track named "Leviathan" on the Destiny 2 soundtrack that contains some Cabal motifs, which would indicate that the raid is going to be a Cabal raid. But with Ghaul dead, maybe we're going to face off against the Emperor?? There was a post-story mission (or adventure; can't remember) that involved decoding transmissions from the Emperor to the Red Legion, so Destiny 2 has already introduced the Emperor as a character, albeit a periphery one.
There was also some concept art for D2 that showed a massive space worm (larger than a planet) with a city on its head. And supposedly Bungie sent out an image of Cabal worshipping one of these worms, drawn so it looks like something you'd find in a biblical illustration.
So, Cabal raid on a space worm?
The physical collectors edition came with a booklet written by emperor Callus, detailing his overthrow and imprisonment. He details the generals who overthrew him, and their weaknesses..... sounds like they might be the raid bosses.