Fan made regions, new enemies, new plants, new pallets, things beyond the void, more LOREEEEEEE, new terrain types (smooth terrain, as seen in the steam page screenshots), and outside of this, new modding possibilities, dev tools, and level editor(s).

Is this true? I think its the devs this time?

i feel like i saw aether ridge in some of the screenshots
it'd be interesting to see that ported to an official DLC kind of like how the old (and better) version of The Root basically became The Rot

The description on the watcher dlc page at the very bottom makes it sound like a new world and animals.