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
