I like the idea of consolidated skills and level-dependent feats, coupled with more feats and few skills considered cross-class. I'd like to see the idea of feat chains scaled back - none in the core books at all, except maybe to illustrate how they should be balanced for expansion books.
I would like to see more of the base classes from other sources rolled into the main game, mainly the Knight, Duskblade, Spellthief, Warlock, Swashbuckler, Favored Soul and a spontaneous nature spellcaster that isn't the Spirit Shaman.
The touch attack to start most special combat actions should be dropped, but the rolls should be considerably harder for the attacker. They should never require more than two rolls per action (a third if damage is applied), much like an attack or spell only generally has two rolls. This would speed up play quite a bit for some of my group, and might encourage others to actually, y'know, use those options.
I'd like to see Prestige classes cut from 10 levels to 5 levels max, with prerequisites at each level. Other than that, I can't think of much they can do to improve the DMG. Of course, I thought the same about the 3.0 DMG, and I was wrong there. Maybe an appendix with "Monster Classes" and a menu of special abilities so that monster creation is easier for DMs.
Given the current evolution of the presentation of stats, I'm afraid of what a new MM will look like. The current stat blocks they're playing around with are easier to use than the condensed blocks, but harder than the 3.5 Monster Manual. If they go with the expanded blocks, I'm afraid we'll have a Hackmaster-like encyclopedia set. And no, I don't want loose-leaf monster folders, unless each set comes with a CD/DVD so damaged sheets can be easily replaced.
It'll never happen, but I'd like to see the core books expanded into Player's Handbook, Dungeon Master's Guide, Monster Manual, and two optional books: Spellcaster's Handbook, and D&D Campaign Guide. The first would reprint the rules on magic, spells, and crafting items from the PH and DMG. It could also expand on these, as well as providing full stat blocks for summoned monsters, familiars, and animal companions and detailed rules on polymorphing (if it still exists!). The second would be cover the Greyhawk campaign, with appendices to cover campaign-level rules: war, rulership, factions, etc.
Oh, and I'd like to see them come up with a new look-and-feel for every other company out there to mimic for the next few years. The sculpture covers were very striking in 2000, not so much now.
I'm ambivalent on the OGL/d20 thing. Keep it or don't. I've pretty much stopped using 3rd-party stuff over the last couple years, unless it's a game system all to itself.