Well, what I'd ideally love...
Planar races - Gith (Yanki, Zerai, Athasian, Wildspace), Rogue Modron, Tiefling (with variants for Abyssal and other bloodlines)
Monsters - Gehreleth/Demodands, Obyrith, Guardinials, Primal Spirits, Abyssal Dreadnought, Atropal, Farspawn.
An in depth guide to the planes: The Primes, the Elementals, the Para Elementals, the Quasi-Elementals, the Positive, the Negative, the Shadowfell, the Feywild, the Ethereal, the Astral, the Far Plane, the Dream, the Mirror, the Temporal, the Good, the Neutral, the Evil, the Lawful, the Chaotic, even the soding Underdark.
Then you could have a chapter discussing cosmologies. The Great Wheel, the World Tree, the World Axis, the cosmologies of Athas (The Grey, the Black, etc) and of Eberron (all those dragons), ones based on mythology. And guides to creating your own, incorporating some planes, excluding others, and merging some together to make a universe that's your own.
And maybe redoing the Modrons to give them some sense of being almost mechanical angels out to bring order to the cosmos. All too often they're just a joke.