Is this a trick question?
From what we know, all the obvious stuff that would in a DMG...is in it. I guess we might not have pages detailing hirelings, guidance on making ravenloft style 3d perspective maps, critical hit and fumble tables, a weather generation system (though we might actually have a simple one), an extended discussion of governance and social class (again, you never know), rules for modern/future vehicle combat or radiation damage and mutation, psionics (sort of in MM, and actually could be something in the DMG), rules for "epic levels" (again, can't be sure)...
We may see this stuff at some point somewhere. Though probably in the context of particularly "story". A set of adventures for Dark Sun could add psionics, more on mass combat, gladiator fights, harsher survival rules, dramatic crits, and so on.