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Is it probable? Yeah, based on the UA. But we do know a lot of material ends up on the cutting room floor, so we don't actually know if there is a dragon book that will be published.
Anyway, I personally don't think a Drow book is the next adventure. It could be, but I do feel like WotC is trying to release very unique adventures every year, exploring diverse environments. We already have
Out of the Abyss, I doubt we would get another Underdark focused book.
I also really doubt this is a setting book, because Drizz't is extremely Sword Coast focused, and that book already happened.