I'd be suprised if it wasn't a Cabal based raid. Location wise i would like to think earth .... for the following reason .. I think they (Bungie) could change it up a bit, instead of us Guardians assaulting a planet/ship/realm we could instead Guard our own turf from an advancing force. As for a new sub class, i think it would be to difficult to try work in another one for now. Ps. Ship battles would be interesting :)