grimslade
Krampus ate my d20s
I don't know that there will be new subraces. I think at most they will create lineages like in Ricky's Guide to Spookytown. The Southern elves live in a 'place of shade', I would imagine this a part of the deep forest/jungle where little sunlight pierces the canopy. I also imagine the Starlight Enclave is far enough north that it is in perpetual night much of the winter. Clothing to combat the cold would also combat the sun.Traditional Underdark Lolthian drow are now called Unudrow, Lorendrow are a southern surface eco Drow civilization, and the Aevendrow the Northern Seldarine Goodly surface Starlight Elves (this will create confusion with Star Elves).
I will point out these aren't the only Drow cultures in FR, there are Maztican Drow who worship Maztican Gods and Drow in Zakhara who worship Zakharan Gods (like almost everyone in Zakhara does), both groups are from previous editions and likely don't feature in RA Salvatore's novel Starlight Enclave.
Not going to lie, I'm very intrigued by this.
Keep an eye out for a Drow UA IMHO, they will likely want different stats for these new Drow cultures, Sunlight Sensitivity makes no sense for the Surface Dwelling Lorendrow and the Aevendrow.
Drow are the elves that turned against Corellon for being an absentee god. Those that didn't follow Lolth were still cursed by Corellon. They just don't get the demon orgies, spider theme, and cutthroat social Darwinism.