There is no evidence from Larian themselves that they care about anything beyond the game they have made already ( which is very enjoyable, thus far, if still lacking much logic as to when dialogs trigger ).
Unlike NWN2 which provided 3 different stories that between them provided complete coverage of the then-current rules set, along with DM facilities, and development tools for players to make their own adventures, Larian seem content to provide 60% rules coverage, and no DM tools, although they have talked about modding tools.
It's possible the success of BG3 ( it is likely already the most successful CRPG made to date, only 3 days after release ) may cause Larian to re-evaluate their plans, but it would still be challenging for them to match BG3's quality in any expansion.
After BG3's bugs are fixed and a difinitive edition produced, I think that modders are the best bet for further development, although you should not expect the cinematic depth that BG3 exhibits.
