Planescape 3.5 Hardcover one-shot ala Ghostwalk
Likewise for Dark Sun
The Rest of the Realms: Faerun’s other regions all in one harcover: Maztica, Evermeet, Zakhara, etc.
Greyhawk campaign setting.
Fiendish Codex III
Celestial Rolls, volume I: Aasimon (we’ll see if it sells well enough to warrant Guardinals, Archons, Eladrins, etc.)
Dictum: Being a guidebook to the realms of Law and their denizens.
Freedom: Being a guidebook to the planes of Chaos and inhabitants thereof.
Adventures published in premium fashion with non-random minis and climactic battle terrain sheet.
Player’s Guide to . . . for Eberron, Greyhawk, Forgotten Realms, Dark Sun, and Planescape, like the one published for Monte Cook’s Ptolus: distributed free as a PDF and at cost in print. Instead of another sourcebook, this should be a slim intro to give an idea what this world is and isn’t in contrast to every other world.
Myriad Worlds: a collection of very succinct campaign worlds and adventure hooks (two pages or less each, with illustrations)
The rest of the environment series: swamps, mountains, et al.
Historical Cultures for D&D: Eight pages on Greece, Rome, and Classical Adventures, Eight on India, Eight on China, Eight on Japan, Eight on Middle-
Americans (Inca and Aztecs together as though they were one or as co-extant) Eight Pages on Arctic Circle tribal hunters, Eight on Vikings, Eight on Celts, and Eight on Southern African adventures (like in Nyambe). This is only 64 pages and could be one of a DM’s greatest resources ever on architecture, variant classes, exotic spells, linguistic differences, whatever. A grab bag of crunch attached to historical fluff. All of the flavor text is already available, but a prestige class here, a spell there, and some exotic delicacies there can enrich an adventure, or even a campaign, better than yet another combat encounter with a fairly innovative monster.
Monster Manual V
Incomplete Player’s Handbook Series:
Complete Races: a collection of previously published and new PC races, including those from the player’s handbook and the various subraces thereof, and including the races featured in the races of series, with paragon and racial substitution classes. Possibly include a pantheon for some, or a race-exclusive spell-list, or other cool feature.
Complete Classes: Fighters, Sohei, Warlocks, Psions, Scouts, Wizards, but also Bear Totem Barbarians and flying squirrel style monks, celestial-blooded sorcerers. Every WoTC class for 3.x and variants for most of them. Perhaps include some highlight third-party classes like the Yogi.
Complete Skills and Feats: a compendium of skills from the PHB and splatbooks with variant and epic applications and a complete list of WoTC feats with a discussion of how to design, limit, and implement feats and skill uses in one’s own campaign. This would include a reprint of the skills portion of Unearthed Arcana, with simpler and more complex skill systems and a new discussion on the impact of increasing or decreasing the number of feats a character gets and the different types of feats like “only at first level” feats like the talents of AE and the feat trees like those used in Iron Heroes.
Complete Equipment: a new Arms and Equipment Guide with more options for magic item creation, more comprehensive random treasure generation tables and pre-bundled adventuring kits for first level adventurers to expedite the shopping for beginning PCs.
Complete Spells: A well-written collection of all of the 3.x spells. This would clearly be a multi-volume tome, so it could be divided either alphabetically, as the AD&D Wizards Spell Compendium and Priest’s Spell Compendiums were, but from a single complete spell list, or could be done by class spell list. Because of the immense overlap and the sheer number of minor casting classes, though, I believe that an alphabetical list is the best way to go.
Complete Magic: This one, I actually wouldn’t publish now, but it would have been what I would have done in place of the new Tome of Magic: A discussion of how magic works and a comparison/contrasting of the various magic systems in D&D and an examination of how they might be broken down and put back together in interesting ways (i.e. What if paladins got invocations instead of spells? Which is more powerful: spontaneous casting from a short list or prepared casting from a long list? What would a bard look like if his spellcasting were Intelligence-based? How would one build a power-slot psion instead of a point-based one?)
Complete Player’s Handbook Toolset: An expansion of the Description, Combat, and Adventuring chapters, this one, like Complete Magic, would get down to the “nuts and bolts” of the system and examine the rationale behind various rules (e.g. Why doesn’t falling damage incorporate acceleration and terminal velocity? Why do quadrupeds get better carrying capacity? Can I safely ignore AoO and forbid grappling, or will that change my game? How do I change/fix/ignore/eliminate alignment? What do I really need when designing my own pantheon? Why would a PC not just retire to blacksmithing/begging/casting Wall of Iron? Can I change the disabled, dying and dead thresholds? What are combat zones and environmental hazards.)
(In house) a workable plan for designing D&D4e for release at GenCon 2012