There are seriously a LOT of tracks we've heard, even in the first season worth of the anime, that we haven't seen on an OST or op/ed single yet.

I'm sure we'll see an OST 3, and probably OST 4 and 5. We'll probably also begin seeing things like "Character Singles" and "Image Albums" -- which will have stuff the core OST's don't have. It's just up to the producers how they want to create and release that stuff, which is anyone's guess.