Fluff books are great, but they still need to appeal to the iconic D&D activites of 1) killing things, and 2) taking stuff.
To that end, I propose the following titles:
Iron Fist/Velvet Glove - One of the fondest memories I have from my old school D&D days was reaching a high enough level to build a keep, attract followers, and really start making a name for my PC. This would be the expanded 3.5e version of that concept. How do PC's establish a Keep on the Borderlands, or a Tower of Inverness? What are the laws/customs that govern this behavior? How do I deal with these followers I have attracted? More importantly, how do I transition from a rootless wanderer looting tombs and ruins into a ruler of men? What does it mean for my character when he treads his sandals on the jewelled throne of a King?
City of Intrigue/Village of Plot Hooks - Take one city, village, urban environment, and completely depict it in fluff. Give the reader a solid sketch of every character that can be encounters. More importantly, give that character's background and motivation. Every NPC a player encounters in this city should be a potential source of encounters. This is essentially an update of the Village of Hommlet. Make every villager, every merchant, every drunken bum a treasure trove of plotlines. Maybe that shabby drunk is actually a minor noble in hiding for some secret shame? Maybe the local druid is engaged in a secret war against the lumber mill? Go the deluxe route with this, give every NPC/group a picture, and at least a one page write up of their plots and agendas.
Chaos & Order - Something along the lines of Lords of Madness, take a look at the iconic races of Law and Chaos and flesh them out in full detail. WotC kept the Slaad as IP, here's the book to finally use them. Let's see the Modrons, the Inevitables, the Bralani, Chaos Beasts, etc.
Big Book of Intellectual Property - WotC kept a handful of iconic monsters as IP when they released the SRD. Some of these (Beholders, Mind Flayers) have received a lot of love. Others, like the Slaad, Displacer Beasts, etc. could use some more action. If these critters were important enough, and iconic enough to be instantly recognizable as "definately D&D" then they should be getting a lot of attention. Give these underserved IP monsters some attention, flesh out the habits, plots and schemes of the Kuo-Toa. Where do Displacer Beasts come from and what do they want? Does the Carrion Crawler just represent the larval form of a more terrible foe? If these critters scream D&D, then put their boots in the fire and make them SCREAM it!
The Far Realms - There's been some great work done in Dragon in the recent past with the Far Realms, and Heroes of Horror touched on it as well, but this is an environment that has a literally limitless potential. Players dig the Lovecraft connection. Flesh out the common types of encounters with these madness inducing intrusions into the material plane. Give us more information on what drives alienists to reach out to the tentacular horrors from beyond. Page after page of flesh and mind warping madness, and ways to combat the same.
The Art of the Deal - One activity that players like almost as much as "Killing Things" is "Getting Stuff". Take the old Aurora's Whole Realms Catalog, mix in the Mercane, and produce an entire book about buying and selling goods, and becoming a wealthy capitalist pig. What types of trade goods are welcome throughout the D&D worlds? How are these goods moved? Aurora's catalog mentioned Ring Gates, Teleporting couriers, and all sorts of other great plot hooks for adventurers. Recent books have covered how to simulate a PC owned business, let's expand upon that. Give the players more than a random profession roll to simulate their bar making a few GP. Let's have details on how to become a shipping magnate, monopolize a trade route, and corner the market on a commodity. Let's have some details on war profiteering, arms dealing and smuggling. Let's have a list of literally hundreds of dangerous to procure, but always in demand luxury goods. Let's talk about financing colonies to take advantage of the natural flora and fauna (ie. The New World). Getting rich is a popular PC goal. Let 'em have it in spades with this book.
If there's still room in the schedule once those are done, I guess we can fit in a Giant book, a Fey book, and a Yugololth book too.
