The question would WoTC/Larian allow a full level 20 build in BG3 with all the DLCs? Or they going after BG4?
Nothing has been announced regarding future DLC plans, nor a sequel.
Traditionally, Larian doesn't do DLCs - what they did in D:OS1&2 is to continue working on the game after release, porting it to consoles, and releasing free "Definitive Edition", with improvements and extra content. BG3 at release might be missing some expected features like DM mode, so that is also something Larian might want to spend their time on post release. That said, it is a bit of a different project then D:OS1&2 - it's not crowdfunded, so budget might be a bit more flexible, and they have WotC backing - and with a massive development team, it is not impossible, that they could find enough people to work on new sellable post release content, while still pursuing their expected goals. That said, Larian's RPG being one price complete experiences from the get go, has been something I have been appreciating a lot.
As far as sequels, Larian seems to favour stand-alone titles. If they do BG4 I doubt it would be a direct sequel, like BG2 was. That said, if they did a follow up, and kept the engine, they could reuse the work they did for BG3 and add more classes/subclasses and more levels.
Anyway, that's all speculation. WIth how much work Larian has ahead of them, I would be surprised if they were thinking that much ahead. They tend to be throwing all they have into each project that they work on. Worked our really well for them so far.