Evenglare
Adventurer
Some sort of cosmology guide. Half the book would go into the different basic D&D planes and the other half would be how to create your own gods, planes, and how to create unique world properties in the cosmos via mythology, for instance take a look at the nordic mythology explaining how Jogmundar wraps around the world keeping the seas from spilling out, and other stuff like that. Cosmic ships would be nice as well.