I don't expect anything beyond fixing and rounding-out the game, because that's what Larian usually do to create a definitive edition. There will be plenty of mods from other sources, however.
The interview with Swen linked earlier in the thread ( assuming it's the one I think it is ) mentions that they are already starting their next project, which may be Sci-Fi from the wording he used. Swen also addressed the idea of levels 13-20 by saying that would be a completely different game, not something that could easily be incorporated in BG3, but gave no indication that they intend to produce such a game.
I think the only reason Larian would do anything beyond a BG3 Definitive Edition is because WotC provide them a compelling reason, which may happen if enough money is made by BG3, but I wouldn't bet your hat on it.
