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.
It would also help if instead of making memes and daydreaming about the companions people would instead more loudly complain about the unfinished state of act 3 and bad story/ending which would force Larian to do something to save their reputation.
But as long as there are people who constantly shout down any criticism of BG3 and instead flood channels with their Astarion fanfics nothing will happen.
