FULL PAGE GIANT FONT BOLD on PAGE 1 (before anything, even the Table of Contents)
: "All of the material in this manual are options that can be easily added to enrich your Dungeons & Dragons play experience. Everything in this manual, including alternate "rules" or ways of using "rules" established in other Dungeons & Dragons manuals are
guidelines...it is afterall, the
Dungeon Master's Guide. They are suggestions you may or may not want to use or explore at your particular table. The material in this book should
not, under any circumstance, be considered to be required or automatically integrated to play the game.
You are the Dungeon Master, use the material herein at your discretion."
Following that...
I think it's a fairly big given (we've been told) that setting specific PC races will be included.
This leads to the reasonable assumption that setting specific classes (lookin' at you artificer) will also be introduced.
We already know Death domain clerics will be there. So it is a reasonable assumption that a few other domains for "bad guy NPCs" would be there. Given there is, specifically, a "Light" domain in the PHB, I expect we will see a "Darkness" [general evilness] domain if nothing else. I would think they'd do a good 3-5 since the PHB had 7.
Magic Items and, hopefully, magic item creation (both for PC crafting and for DM "make it up") guidelines.
I expect no fewer than 3 appendices of "Random Tables": Monsters (by HD/CR, by terrain, by creature type). Treasures (coinage, gem/jewelry, magic items, art objects, rare/expensive materials, etc...). Dungeon Dressing (self explanatory, but I would like to see individual tables for, say, actual Dungeon Crawl style Dungeons, [lived in or once lived in] Castle/Keep Dungeons, Tombs/Temples, Ruins, etc... What one might find in various types of dungeons should/could vary greatly.)
"Expansion/optional module/guidlines" for "Tactical Combat" style play.
"Expansion/optional module/guidelines" for alternate Magic mechanics/systems.
"Expansion/optional module/guidelines" for more "narrative/free form" play.
"Expansion/optional module/guidelines" for various ways to handle/integrate deities & world religions (polytheism, monotheism, duotheism, animism, extremists & zealots, gnosticism, mysticism, etc...) This could be an appendix too/instead, I suppose.
Dungeon/Campaign creation guidelines. Including a nice list of "generic" plot hooks, "jumping off" adventure ideas and (hopefully) [at least] 1 small/simple sample/"starter" dungeon that can be run through as a quickie (single session) game...with suggestions of where to, organically, take your players next.
World-building system/guidelines.
Explanations/Incorporating adventure ideas of Political/World power systems: democracy, magocracy, theocracy, feudalism, imperialism, etc. etc. etc... (Possibly could be wrapped in/included in part of the world-building stuff).
General "How to DM...[for the PCs]"...likely broken down into Combat, Exploration, Interaction. Also general guides for stuff like, "How to Set a Scene", "Knowing your Players and their PCs/Player Expectations", "Making Monsters/Encounters interesting", etc. etc. etc... Sounds like a lot but none of them (other than the 3 pillars, I'm guessing) has to be particularly long. A paragraph or two of generic "FAQ" kinda "how to's".
General "How to DM...
At the Table" type guidelines/ideas. How to handle conflicts among players. How to handle conflict
with players. How to handle chronic absence/tardiness. How to handle hygienic issues. Just some basic interpersonal communications type suggestions.
...that's all I can really think of (at the moment) that I would hope to see in the DMG. Nearly half of this material is not stuff I, personally, necessarily need or would use. But think it would make a "good" DMG...and I'm going to buy it anyway, whatever it has. So don't really go by me.
[EDIT
I forgot...
"Expansion/optional module/guidelines" for stronghold building (castle, trading/guard outpost, thieves' hideout, wizard's tower, full temple complex, abbey/monastery, "typical" elf/dwarf/halfling "clan-style" holds, etc...), settlement and dominion creation/management.
"Expansion/optional module/guidelines" for mass combat.
So add a few more pages for all of that.