Personally, I want to sit in on the meeting where you tell Oprah all about
Fiendish Codex III. Throw in
The Book of Erotic Fantasy II and
The Book of REALLY Vile Darkness too. (Not that I've got no love for the yugoloth - I'd just like to see how Oprah takes in the whole explanation of the Blood War.)
Needless to say, most of the ideas for things I'd like to see (and write) have already been suggested... some many times. I'd love to work with Nic Logue on a Heroes of Intrigue book, or to have something to do with a general book on Fey. However, since Eberron is my thing...
Planes of Eberron. As others have said, this is one of the big holes in the setting. We step away from the Great Wheel, but then we don't have a lot of information about what's out there. The Planes of Eberron affect the world in many ways - manifest zones, wild zones, planar convergences, and the constant threat of incursions, to name a few. People can end up on other plane just by walking through the wrong grove of trees when the stars are right... but this isn't much use if the DM doesn't know anything about the planes. I'd love to look at the culture and history of each planes, the nature and motivations of their inhabitants, and how the planes relate to each other.
Cabals and Conspiracies. A closer look at the hidden powers that shape Eberron. The Lords of Dust, the Chamber, the Aurum, the Dreaming Dark, the Lord of Blades, the Undying Court, Vol, and their ilk. The history behind these organizations, their motives and goals, and lots of ways to use them in a campaign, either as enemies, allies, or even the basis for a party of adventurers. You
are Lords of Dust - do you have what it takes to outwit the Chamber and free your overlord?
Ages of War. Forge of War provided insight into the Last War, and helped people see how to run campaigns during the conflict. I'd like to look back at the great struggles of history, and let people step into each of these battles. In each of these cases, the goal would be to provide an alternate campaign path in the past of Eberron - but also to provide material that could be used in a modern campaign.
- The Lycanthropic Purge - do you play as templars struggling against a terrifying foe? Lycanthropes - either good 'thropes trying to undo the damage and prove their innocence, or evil 'thropes using cunning and supernatural might to destroy the forces of the Silver Flame? Or the shifters caught in the middle?
- The War of the Mark. Whether you take the side of the dragonmarked houses or join Lord Tarkanan and the Lady of the Plague in their struggle for survival, you will learn more about the struggle between the aberrant and the pure. Beyond the potential for a historical campaign, the information about the power of the ancient aberrant marks and the weapons and focus items developed by Tarkanan and his allies could play a significant role in a modern-day game.
- The Fall of Dhakaan. The Dhakaani Empire during the height of the Daelkyr Incursion. Hobgoblin heroes and orc mystics struggle against a tied of madness and things that should not be. This would provide an opportunity to explore the Empire of Dhakaan, but also set the stage for what might happen if the Daelkyr are released in the present day.
- The Last Days of Xen'drik. The Valenar of the modern day worship their ancestors, the warriors and wizards who rose up and battled the mighty giants. As an elf in the darkest days of Xen'drik, do you have what it takes to become one of those heroes, to inspire your people and challenge the giant overlords? Beyond the war campaign, this would provide a great deal of insight into the modern Valenar and relics that could be found in Xen'drik.
These are just a few ideas. One could look at the Sundering in Sarlona, though it's really too broad. The first Quori Incursion (which would presumably involve playing either giant or quori PCs). Galifar's conquest of the Five Nations. In any case, the goal would be to look back at some of the major events in the history of Eberron, to offer an opportunity to play in these periods, and also to see how they can be used in a modern campaign.
The Rest of Khorvaire. Yes, it would be nice to have
Five Nations - style books covering the rest of Khorvaire. While one book would be better than no book, it really wants to be two books - East and West, or Wild and Civilized. Either way, I'd love to see either of them.
I'll stop there, but there's lots of other possibilities I'd love to see. The Khyber book suggested earlier would be fun. "The Secret of the Mourning: Revealed!" - a 160-page book containing 160 possible explanations for the Mourning. OK, maybe not...