Back to a topic at hand, my guess is that Larian will make DOS3 first, but until that happens, they'll support BG3 for a longer while, by fixing and tweaking things and possibly adding new races (aasimars, please!), classes/subclasses, spells etc. I'm not sure about an expansion, though, that would probably depends on sales numbers and how badly WoTC needs money.
I don't imagine they won't be tempted to expand the story, so we could achieve levels closer to 20, if only for the fun of making all those fancy high level spells, but they are not exactly known for having a track record of story expansions, so we will have to wait and see.
