Personally, what I would like, is a book where the setting is outlined in a small number of pages, similar to the DMG but a number of more detailed locations for each setting, accompanied by a bunch of one-page adventures.I do not think 5e24 needs a setting book.
Forgotten Realms works well enough, however I would not be averse to a Book of Settings where one could compile and expand on those interesting MtG settings (or make new ones) that came out early on in 5e's history.
Those were pretty cool and unique (not every race was represented in each setting) and there was great art.
As someone who does not play and follow MtG I absolutely loved those.
May themed about some adventure premise. Perhaps a low level planar portal based adventures with a number of prime and planar locations with adventures and maps.