A planes DLC would be great, Planescape Torment would be an inspiration source. Building a Baldur's Gate-level of quality Sigil would be terrific, but maybe that's for BG4 instead of a DLC
If you're willing to put the resources in, a seafaring campaign with ships, pirates, Umberlee, buried treasure, swimming and the like (yo... crazy idea, but level one Balduran prologue?).
A Gond campaign (jungle, dinosaurs, artificer joining the roster, pyramids, mummy lords, etc)
A lot of people want Curse of Strahd. I want Curse of Strahd too
Curse of Strahd-like DLC! Get into that vampire stuff for real, not like Act 3!
Not a DLC per-say, but releasing a creation kit and DM mode (like D:OS2 had) would be amazing for people creating custom campaigns, and then you could curate some and include them in the game (like Beamdog did for NWN1). Considering the great success of BG3 and the sheer number of people owning the game, there might never be a better time to try DM mode again...
