Though I'd eventually buy a planar book (especially if it has monsters), it's not something that particularly excites me. If I had my dithers, it would be the Planar Sourcebook similar to the 3e Manual of the Planes make your own cosmology approach with the 5e's (version of the) Great Wheel and 4e's World Axis as fleshed-out examples. However, the realist in me thinks that we would get either something more like the 1e Manual of the planes that only details the Great Wheel (but gives Sigil some lipservice) or a Planescape-focused book that highlights Sigil and expands upon the planar information in the DMG. I also expect subclasses, races, monsters, maybe a handful of spells (planar and summoning related), maybe backgrounds (especially if Sigil-focused), maybe feats.