I believe there's a bunch of licensing hurdles involved, so I don't imagine Larian will go out of their way to include much beyond the PHB and a couple very popular things beyond that, unless WotC is making stuff precisely to tie into BG3 in some way or vice-versa. WotC has a notorious iron grip on the licensing and legal aspects of everything to do with the DnD IP.
The Solasta devs in particular outright said the reason why their classes and setting were so heavily homebrewed is because they only had access to the open license. And they are also outright discouraging people from modding in official content or re-creating actual modules through their dungeon maker, or else they could get in a lot of legal trouble.
