Like what Fizban’s Treasury of Dragon’s and Bigby Presents: Glory of the Giants did for expanding the lore of their respective creature types what other books would you find interesting and useful?  Previous editions have had undead-focused books and Van Richten seems an obvious ‘product identity name’ to put on one.  The Demonomicon of Iggwilv could expand fiend lore.  Fey lore could use some expanded lore options, but who would present it?  Creature types could be treated as tags (or conditions) rather than exclusive categories, so there could be creatures which fall into multiple groups, and the catchall shrug-bucket of ‘monstrosity’ could be retired (why is a stirge a beast when an owlbear isn’t?  Why isn’t a mimic also an ooze?).  As is done with the shapechanger tag, mechanics could be streamlined by making tags function like conditions; a ‘fire’ tag could apply to some elementals, some dragons, etc. indicating that they’re immune to that damage type.
Practically every book has player options these days, but DMs still buy the most, and we need tools for making the prep and running of games a bit easier, to keep up with some of the more the powerful species, subclasses, and spells.  An easy way to scale monsters up would help.  We have some reduced-threat monsters, but increased-threat would be more valuable.  Here on DDB, you could even make creature creation work like character creation, so we don’t have to mess with code to make an upgraded wyvern or whatever.

Silly (putty) creature types might have lorebooks that are silly, but Jubilex’s Splatbook: Secret of the Ooze could work.