So Mike Mearls is on the phone...

[pkt77242]

Dragonlance campaign setting. Set around the War of the Lance.

 

[guachi]

I'd do something similar to the OP, though I'd make it an adventure where you travel from setting to setting for each Act. Maybe use Planescape or Spelljammer to travel to the older settings of D&D and you could include a little background for each of the locations.

 

[rgoodbb]

Dark Sun or Al-Qadim. We definitely need more sand for our boxes.

 

[Guest 6801328]

"Mike, you don't pay any attention to the self-proclaimed Hard Core Fans on places like Enworld, do you?"
"Not a wit."
"Ok, good, just checking. See ya."

 

[Older Beholder]

'The Monsterous compendium'
- 700 more pages of monsters.

 

[Count_Zero]

Big Book of Dragons: like Volo's, but all Dragons, all the time.

I like this - but with a twist - doing this as part of a Council of Wyrms campaign - setting information on Dragon society, more information on different types of dragons, rules for creating and running dragon characters, and a campaign that you could run with those characters.

 

[77IM]

Van Richten's Guide to the Horrors

Weird... I was imagining this exact same book earlier today! Same title, roughly the same content... this indicates to me that it's probably too good of an idea to ever actually happen. I'd probably include some character-creation options (e.g. monster-hunter subclasses), and a chapter for the DM on running horror games, since previous WotC forays into this territory have proven extremely fruitful.

But if I had Mearls on the phone, the book I would pitch would be...

Savage Species
160-pages
Price: $39.95

Rules to play all sorts of monsters, not just 1-HD humanoids. Kind of like the 3E Savage Species, but not sucky and terrible. It's notoriously difficult to balance monsters as PCs, but there are numerous DMSGuild products that tackle this issue, some with surprisingly good results. Heck, I made a dragon character class and it didn't turn out too bad.