Larian started work on DD and BD with relatively small teams, then added a few programmers, then added artists and writers during the course of development. Assuming they are doing well, and actually want to, they could have the programmers start on the game engine for Divinity 3, while the writers and artists, etc work on an expansion.
As for what I would like to see in a hypothetical addition to the game that I don't have yet... access to more of the regions from DD (though obviously they would have changed), contact with any surviving elves, dwarves, etc. Depending on what Larian has planned for Divinity 3, though, this may or may not be feasible.