Set, I come up with the idea to just have a group of "domesticated" dragons. Give them a variety of body shapes and abilities kind of like How to Train Your Dragon did.
That reminds me of the Dragonblooded creatures, found in D&D 3.5 Dragon Magic. They were literally animals and vermins with draconic traits, but not "enough" to treat them as true Dragon-type creatures.
Back to material, I would have loved a Soldier Codex:
* War tactics for each core race
* NPC stats for units and ranks (footman, pikeman, archer, scout, knight, mage, healer, etc)
* Archetypes, feats, items and weapons based on warfare
* Humans, elves, dwarves, halfling and gnomes as core races being represented
* Leaving half-elves due to be with either humans or elves
* Leaving half-orcs due to be with either humans or orcs (previously stats in the Monster Codex)
* Essentially a Monster Codex II, but with an emphasis on core races