I'm thinking monster book (like Volo's/Mordenkainen's) with both Dragon and Feywild material.
It would fit the mold well. A Dragonomicon would be pretty small in scope for a 5e book, but a Dragon + Feywild book combined would match the varied material in Mordenkainen's. Having both new monsters like the Sapphire Dragon, and new Fey, plus the Fey and Dragon themed players options.
It meets several criteria for me;
1. Haven't had a monster book in a while.
2. Is different than the confirmed 2021 releases (Adventure Compilation, Setting).
3. Fits neatly in July, between Ravenloft (May) and annual adventure (September).
4. Covers recent UA.
So I'm not certain, but it makes more sense than other things I can think of.