Spelljammer is way too big to put in that DLC, even just Realmspace us really big.
I think your Underdark idea is more likely, or another region of Faerun, but I struggle to figure out what region of Faerun would justify 7 Species?
Kara Tur is huge, 10 regions, although most are a single nation, with a few exceptions.
What do you think the page count for the DLCs will be? If it's 50 pages minimum maybe.
I still think Kara Tur justifies a book of its own. Even Al Qadim is more likely, it's just 1 big empire mostly. But perhaps that's stepping too much on Calimshan's toes too soon.
Unapproachable East is more likely honestly. It's the end of the Silk Road so Kara Tur species like Spirit Folk and Hengyokai end up there, but you also get feywild stiff, and maybe experimental creatures from Thay? I could even see Genasi showing up given in ancient times an Avatar of Kossuth was summoned to the area.
Maybe Old Empires Region? I can see Reborn and a weird twist on Mummies, Genasi of Akanul, close enough to the Shaar maybe for Wemics & Loxo. Satytrs and Nymphs in nearby Chessenta.