To me it seems like undead and fiends are the broad categories with enough variety, enough thematic coherence, and enough creative space to design new entries to sustain a Fizban's style book.
While expanding out fiends to also include their goody two-shoe cousins the celestials, or to narrow them down into separate devils and demons books makes a lot of sense to people fully initiated into D&D planar lore, I think in terms of being able to also sell to a more casual fan base and to people who just don't really use the official cosmologies, a book of devils and demons is probably more on the money. That said, supporting Planescape may throw a curveball into that calculus.