I think a few people are setting themselves up for disappointment...unless the DMG is a 400-page behemoth!
I want to see solid battle-mat combat options that can be freely mixed-and-matched with the existing totm combat described in the PHB, so that the only time the battle-mat needs to come out is for major opponents or more sophisticated environments.
I would like to see the 5E designers' take on skill challenges. Not a re-tread of 4E's arguably difficult rules, but a framework for complex, multi-action group problem solving. (To be fair it's about five minutes' work to conjure something up already, but I'd still like to hear a few ideas from the pro's.)
Like [MENTION=1465]Li Shenron[/MENTION], I want to see alternative ways to run large combats without a grid: swarms, the equivalent of 4E's minions, that kind of thing.
Monster building rules are an absolute must. I'm surprised they weren't an Appendix in the MM actually (they weren't, right?).
I *don't* want too many pages given over to alternative campaign settings or their races. Seriously, what justice can be done to them in a few pages? But, I can see the value in continued integration of D&D's many worlds into the DMG, as it was in the PHB.
I *don't* want alternate spell systems. That would just make the DMG a splat book by another name.