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.
The financial argument seems solid to me, with some caveats.
Expansions by definition have a cap on sales numbers. You're selling a cheaper product that comes with a prerequisite, for profits to be good you need the expansion to be made at low cost.
I don't know how cheap BG3's engine is to use, but it can't be all that cheap. With all the animated dialogue, the permutation work, motion-capture, what-have-you, Larian are going to need to sell a lot of expansions or make them quite pricey.
It's also telling that the magic mirror is limited to the player character. It seems like any character who speaks has to have their animation match their model to prevent clipping or other glitches. I don't think that kind of limitation is conducive to a quick turnaround.
While I still don't see an expansion coming out, I don't want to rain on your optimism either. If Wizards of the Coast get a say, things are looking up for an expansion: WotC are nothing if not profit-driven. This time, capitalism is our friend!
