So Larian's stance has always been that they'll be focusing on PHB content - though they have definitely bent that rule with player races (makes sense given their story importance).

There hasn't been anything concrete, but there was some huge HOPIUM a while back due to people datamining content for a 3rd Barbarian Class (wild magic) when Patch 7 released. I don't know how reputable Techraptor is, but there was this article:
https://techraptor.net/gaming/news/baldurs-gate-3-datamine-reveals-non-phb-material

I'm not aware of any updates since from dataminers though (I wouldn't be surprised if this was wiped in Patch 8)