I'm torn between two potential ideas:
A Planescape setting, or at least a sourcebook for the Outer Planes.
or
A second Baldur's Gate adventure, given the "Waterdeep:" precedent we had last year. If they want good symmetry, it should be an "Ascent" to mirror "Baldur's Gate: Descent to Avernus". Given then Bhaal imagery on the cover of the book (not just the symbol on the upper left, but you can actually see Bhaal's symbol in the clouds behind Zariel on the actual cover artwork), Bhaal (and presumably Bane and Myrkul, as they are all three physically active in Baldur's Gate from a recent Lore You Should Know) is going to be a major player in this adventure. Perhaps a second adventure will have them attempt to ascend to full godhood, instead of the half-deity/half-mortal state they are now?
Then again, once more citing precedent, we did have two books last November, an adventure and a setting book. Maybe we'll get both again?