3.5 Updates of Monster Manual II and Fiend Folio, or better yet, a "Monstrous Compendium" collecting both books' contents updated to 3.5.
Born of Magic- A guidebook on magical beasts, oozes, and constructs, expanding on the origins and nature of purely magical creatures beyond "A Wizard Did It", and the impact of such created beings on the world.
Primal Legends- A guidebook on giants, fey, and elementals (including genies) detailing their place in the D&D world, and perhaps including a section on the mythic origins of (at least some of) these creatures and how to evoke the feel of real world legends in the D&D game.
Dirigibles & Dragons: The Complete Guide for Steampunk and Pulp Adventure for core D&D, including definitive stats for renaissance and industrial revolution era firearms, dirigibles, ironclads, and advice on introducing such technology into a core D&D world, including options for Magic vs. Tech, Magic/Tech fusion, and Tech only settings. A professionally designed and balanced item crafting system, and new options for the core classes as well as Swashbuckler, Warlock, and Artificer would round out the selection, along with stats for Tinker gnomes, tinker goblins, and a steam-driven warforged variant.
To be entirely honest, I don't think any of these were likely to come out of WotC. Also, before people start pointing me at steamtech supplements from 3rd parties, let me say that I own quite a few, but having a WotC-produced book would go a long way toward validating my preferred sort of D&D setting to DMs and players who only want to use WotC products in their games.
Robert "Steam-Pulp" Ranting