Stepping back into the DLC discussion a bit -- it's true that Larian doesn't really do expansions or DLC. I'm not sure why they haven't been keen to do expansions though (which I define as substantial added content -- for example, compare Witcher 3 DLC and expansions), because I doubt many people would be unhappy about expansions. Still, they don't seem keen. I suspect maybe they just prefer having an opportunity to reinvent the wheel a bit regarding game mechanics.
I don't see any reason why Larian wouldn't be willing to do a sequel linked to the original story and a higher starting level unless they feel it would make for a difficult entry-point for new players.
Anyway, there's no reason to assume the level cap will definitely be level 20. Larian isn't making a virtual D&D tabletop. They're making a Baldur's Gate game first and foremost. Obviously they are trying to mimic tabletop D&D but they're still making their own design decisions for their own game. I think they're more interested in making more of the weird low-level stuff as good as they can get it, rather than watering it down so they can pump out all 20 levels just for completionists. (Much like what dwig said.)
