Two or three each of new (or slightly modified) core classes, prestige classes, feats, spells, core races, magic items (with prices), etc. Each with explanations of how they were built, non-obvious loopholes and other mistakes that were initially made and later corrected, minor pointers on how to fit it into your campaign, and the like. Maybe one or two examples of things that almost made the PHB/DMG/MM but didn't. As well as more guidelines for creating/modifying what you have overall.
Genre modification ideas and house rules. There's been enough interest in a VP/WP system, if it ports well it'd be nice to see the D&D equivalent. Ideas for running low or high magic campaigns, beyond the blurbs in the DMG, as well as running games that range farther afield (removing a core class, no magic, only arcane/divine magic, etc.) Especially guidelines for "if A, than change B to pick up slack". (F'rex, undead and damage in a clericless world, or DR monsters in a low magic world.) Mood-setting tips also go here. Also add adventure locales other than dungeons, and how to deal with the expanded decision tree. Maybe even a synopsis of a newly created world, Dungeoncrafting style.
Advice for working around sticky situations, be they in game (my players always scry their enemies, have a friend teleport them, and cast disentigrate on their normal, haste, and quickened actions. How do I level the playing field without making everyone mind-blanked?) or out of game. FAQ's for tricky parts of the rules are nice, too. But most important for the DM's who need this book is how to deal with problem players/groups. Especially the need to say no occasionally, how to try to resolve the problem without kicking the player out, and realizing that sometimes you just have to break the group up or quit for your own good.
Explanations on the difference between a prestige class, a tweaked core class, and a multiclassed character to match an image. Showing where each works best, with maybe a quick NPC to show how each would work. (E.G: A Wizard with one level of monk as another way of making the active Seeker of Wisdom or a more westernized hero or adventurer monk, a Fighter with Knowledge: Nobility and Diplomacy as a noble knight, and the Dwarf Paladin whos calling lead him to become a Dwarven Defender.) All that would be really needed is level(s), feats, and maybe skills to focus on.
Tips on dealing with (or avoiding) touchy subjects in game. This'll be a short chapter, but priceless for the novice DM. And maybe some NPC/adventure templates, some pointers on different styles of play (one off, short, campaign, extended campaign, goofy, dark, etc.)
