I would love an expansion that added another act (between 2-3 as you fight your way to BG city?) or sub-act (like how the Underdark and GF are sort of an optional sub-act for Act 1, Acts 2-3 don't have Underdark type side maps with contained quests the way Act 1 does it), talk to this boat having NPC and go off and have related mini-adventure that has X effect on main story.
If Minthara can't be fleshed out and honestly, finished as she's only half-done, through updates, then maybe a Minthara Origin with unique prologue where you serve the Absolute then kill 'tav' and become released by holding the prism then meet up with the rest of the origin characters and continue the game normally from there, could pay for the completion and fleshing out of her character and quests.
An Epilogue DLC if they won't do that in patches too, to allow you to explore /see resolutions.
For a full expansion, there is so much potential for OTHER stories on these maps, and setting. Or explore the cloakwood, bereghost or fields of the dead during the events and see what the absolute is up to there. Add more spooky underground temples or wizard towers to explore. Fight the other BG characters 5e lore says are still in BGcity, Stronghold quests!
You could have a whole separate story, with new PC and companions that explores the events from another perspective, i.e. maybe the absolute is also operating in Elturel (taking advantage of the chaos after it's return from hell, maybe the heroes there could discover the absolute were behind the exile of the tieflings as a ploy to gain power? or an unrelated to absolute story set in the post-Descent Elturel?) or Daggerford? Maybe a self-contained dungeon delving expansion adventure ala Watchers Keep/Beneath the Stolen Lands/Endless Paths of Od Nua/etc
With Netheril being essential to the main plot, maybe our heroes go to Warlock's Crypt to learn more about the Netherstones? Explore a city of the dead, meet a new undead skeleton/lich/who knows companion there, foil the Absolutes attempt to gain an artifact of old Netheril that'll make them even more powerful?
Even old short adventure paths could be adapted into stand alone story DLC for BG3, the is fantastic, most assets could be reused, you could even wiggle the lore around to set them during the events of the story to have our PCs explore them (remember the Bhaalspawn didn't just chase Irenicus, they took time off to explore stronghold quests and go out to Trademeet and Umbar and etc. so it's in the spirit of BG to have random side adventures with fate of the world/local community/etc on the line there too.) rather than having them stand alone. Or do like Pathfinder games have and have the side adventure DLC's be playable both as interwoven into the main campaign AND as solo side adventures with non-main campaign PCs?
There is literally endless opportunities for DLC/expansion adventures, it has the whole of Faerun setting to draw from, before you even get to DLC new companions, questlines, origins, etc.
With how successful BG3 has been, you could even have a remake of Pools of Radiance or the originals on this engine? or idk, the sky is the limit.
