Well what I’d really like to see is a setting agnostic generic 2D20 rulebook with lots of ideas on world building and how to modify the rules to synchronize with certain aspects of the game world.
But if I should choose from existing IPs/books, the 1st one that comes to mind is China Miéville’s Bas-Lag books - Perdido Street Station, The Scar and Iron Council. The wolrd is so rich, so colourful and wierd that it cries out for a proper setting book.