Bg4 could do what Bg2 fud in essence. Start in a big city, and work your way out in reverse of BG1/BG3 where the namesake city doesn't appear until relatively late. Maybe Cormyr the country. Perhaps Water-repellent. Heck, with Waterdeep, you'd never have to leave the immediate environs for adventure. Espicially with Undermountain. And, so many ways to connect to a variety if environs. Maybe Silverymoon.
An expansion to bg3 could be Myth Drannor. That could give good use to high level characters if you know it's history though Im not 100% sure what 5e did with the Myth , but a certain enemy type that starts with P could be cool.
Or we can head south to Calimshan/Calimport though some might hate that.
MD is a long way from BG, most of what is there could be done with Warlock's Crypt which is a location no game has done, and is within a weeks boat travel of BG? For expansions, BG is surrounded by locations that'd be fantastic to explore. Elturel to see the post-descent city, Tieflings are banned so it'd be fun to go there as /with Karlach?
Moonsea and Myth Drannor could be a great stand alone high level story? Waterdeep would be great, or an expedition to Chult, exploring the old Zhent lands now they aren't the black empire but a theives guild what sort of evils have their ruins unleashed? Idk, it's faerun, spin a cat and you hit something canon to build a story on
