To my knowledge, Larian have never made an expansion for their games. I expect BG3 to get the same treatment as DOS2: a definitive edition in a year or so, during which time other parts of the company begin development on one or more new games.
Larian has never had such a massive IP in their stewardship before, either. The size and success of BG3 dwarfs every other game they've made, so a DLC is a lot more likely under these circumstances. The financial incentive for a DoS II DLC may not have made sense, whereas it's probably a foregone conclusion that BG3 is getting more content. I'm sure Wizards of the Coast will have a say in the matter as well, since every other BG game has had DLC. I'm optimistic.
