1. Terra Arcana/Where Magic Sleeps/The Tainted Lands: In worlds touched by magic, there will be places and touched by magic, from simple fey rings to deserts of ash, from natural leylines over created zones of elemental dominance to places of catastrophe like the Mournlands. Example regions described in the main text should be grandiose in some fashion (perhaps up to Mournland scope), and interesting places to set adventures. Smaller and/or less hostile places can be described in sidebars or appendices. Ruleswise, provide a number of (flexible) building blocks a la planar traits and a few character options dealing with magical terrain.
2. The Vast Above: Tagline - Not even the sky is the limit. Explore the castles of cloud giants, see the magic of the skies, and find what lies beyond. This may deal with "just" the world of the clouds, which is at least as expansive as the depths of earth and sea, but preferably would go above, and provide cosmology options for what the significance of the stars, the sun(s), and the moon(s) is, and the darkness between. It should stay clear of sci-fi or lovecraftian themes where possible, though (that's what d20 Future is there for).
3. Ages of Fantasy: A common theme of fantasy worlds is that they undergo various ages - the Golden Age, the Age of Magic, the First Age, the Age of Insects and so on. What happens when the End of Times is about, when do ages turn? This book would cover a number of common ages and their impact on people and land alike, with ample material to flesh out other ages and small rules components to plug in.
4. Eyes of the Monster: A close look at the evil humanoids, their cultures and societies, their differences and histories. What shaped their societies to become mostly evil? Are these reasons still relevant? How do Flinds and Gnolls differ from each other?
5. Time and Othertide: What happens when time and space unravel? Where do you go when you are nowhere? How close is the Far Realm? What are good ways to make prophecies work in-game? This book would deal with the impact of chronomancy, temporal chaos, alternate worlds, the astral, ethereal, and shadow planes, and strange places that are seemingly just outside the normal cosmology.
6. Vow and Tradition: A book that looks at several societies, orders, and guilds, some class-based and others open to everyone, some secret and others in plain sight. Provides examples of reworking the flavor of existing classes without (hardly) changing the mechanics, especially for monks.
7. Homes of the Souls: Building and expanding on the flavor of incarnum, this book looks at the Positive and Negative Energy planes both, providing interesting locations both within the planes and outside, also covering planar variants, such as a single yin-yang plane of positive and negative energy.
8. Sciences of Magic: Where does magic come from? How was it developed? Was there a first wizard, and did he or she live before or after the first sorcerer? What secrets of magic lie yet undiscovered? Who came first, deities or clerics? This book would provide a Progress Level-like breakup (but more flexible) of advances in magic and similar disciplines (such as psionics, truenaming), possible origins and destinations, limitations and interactions, and theories on magic.
I definitely second several of the suggestions in this thread, though.
