A book dealing with the relatively newish notion of Substitution levels / Alternate Class features. I love the idea of being able to customize the various classes in different ways, based on race, regional variations, different cultures, etc, etc. Variant classes, such as presented in Unearthed Arcana (or classes that are inherently variable, such as the Witch or Totem-Warrior, of Arcana Unearthed) also work better for me than fairly specialized classes like the Paladin.
At least one book devoted to updating each of the following settings;
Al-Qadim
Scarred Lands
Mystara
A book about uses for skills and how to integrate them and involve them during a game, those forgotten bastard stepchildren that live in the murky shadows beneath feats, spells, etc. (Alchemy, in particular, deserves a book of it's own, or at least a chapter of it's own in a 3.5 Arms & Equipment Guide, right next to the chapter on Craft skills and how to use them well.)
I really, really, really *don't* need many more monsters, feats, spells or Prestige Classes (or base classes). I've already got many, many times more of each of those than I could ever possibly make use of in a coherent world / setting / game.
Books that enhance what already exists in-game would appeal to me rather than laundry lists of new spells-that-look-like-floating-energy-weapons or savage-noble-animal-headed-humanoids or feats-that-let-me-sneak-attack-sentient-vegetables or 10,000-new-ways-to-use-that-shiny-new-Swift/Immediate-Action-mechanic.