IIRC, Swen said that after DOS, rather than make a new game, they will do an expansion first. Basically, the engine Larian made for DOS is their RPG engine... as in THE engine they intend to use for a lot of future projects, not just the one game. So I'd expect to see at least 1 DOS expansion, and then maybe Divinity IV carrying on the story after Divinity II with the same engine as DOS and (hopefully) the same gameplay.