There are so many mechanics and assets this game uses but could be reused for other adventure paths and original adventure paths. Like the bards can perform to making money busking or distract for crime reasons, but you don't get to a town till act 3 when you're richer than midas.
The lower city could be rejigged for other adventures easy, or other parts of the lower city using same assets in different combinations, so multiple adventures could start in BG, have an act investigating a plot/crime/danger in the city, discover it''s coming from warlocks crypt? Mintarn? Elturgard? Troll Claws?, some spooky cursed barrow in Fields of the Dead? Najara? hells Candlekeep!
Little 1-2 act adventures based in or around BG city could be a great way to make use of and extend the mechanics and assets of this game. Either stand alone or expansion style sub adventures (think how PoE did White March or Fallout 3-4 has it's offmap new map side adventures within the main game.)
I would love a stand alone+available in game giant dungeon, like the watchers keep in BG2 or Oud Nua's dungeon in PoE, or beneath the stolen lands in KM. Larian could do amazing things with their puzzle combat in a no story just giant dungeon DLC!
For a sequel though, this game front and centre has Nautiloids, so to me to follow up should be Spelljammer. I don't know what the Dead Three are up to on other planets in the solar system but I think in BG4 Withers should send us to find out and stop it. Especially if that Pirate asteroid colony was somehow involved
I think starting a new party in the city and having some adventures there would be an interesting idea. Larian have already made the assets and everything. It could be a prequel and cover events leading up to BG3 even.
That's be brilliant, having unrelated adventures by occasional dark hints about this absolute group doing something? You run off to investigate the netherese ruins at Warlock's Crypt to, idk? stop a rogue lich from raising all the dead of the fields of the dead to conquer BG? Stopping Edwin from some evil plot? thrawting that cult that wants to make dragons undead? something but while you're doing it, you meet Gorlach and theres a sidequest to help him do some research on this netherese crown. Easy sidequest no consequences right?
You could have wholely unrelated adventures, but then if you think about it after winning or next time you play the main game go, wait a minute, it all ties in!
Also set after the game too, imagine a group of adventures hired to clean up one of the crashed Nautiloids on the outskirts of BG, you press the wrong button or touch the nerves on the control panel only for it to use the last of it's power/life transporting it, and you to some other plane (Ravenloft? Dark Sun? maybe even Sigil from planescape? or just any random other plane) and you've got to find a way home saving the planes while you do it?
