I'd LOVE a book on Mechanus lore, that's an instant buy for me. Warforged are interesting to me as well but aren't they completely unrelated? Might be an excellent book possibility to compile various Construct lore into one volume.
I just don't see much potential for lore about Beasts, but back to my initial answer I could see it fitting really well into a book more focused on Monster Hunting and Harvesting. On second thought though, if WotC had ideas for fleshing out the Beastlands plane and how that relates to beasts (and humanoids?) on the prime material plane, that actually could be near the top of my wishlist
I was on the fence about including Undead creatures / Shadowfell plane as the flipside to Fey creatures / Feywild plane, but was thinking that it seems like DnD Undead go well beyond Raven Queen / Shadowfell. They could probably make an awesome book from exploring Feywild + Shadowfell together, updating from 4e lore.