D20 Modern style talent trees for the 20 Base Classes (Barbarian, Bard, Cleric, Druid, Fighter, Monk, Paladin, Ranger, Rogue, Wizard, Psion, Psychic Warrior, Wilder, Duskblade, Warlock, Favored Soul, Scout, Archivist, Beguiler, and Artificer).
Action Points (ala Eberron, not UA) in the core book.
Figure out a way to make all "Powerful Races" LA +0. Then make racial classes for them for the players who want to have some/all of their former powers.
Player handout to explain ridiculously simple concepts like Attacks of Opportunity and the difference between Encounter Level (EL) / Challenge Rating (CR) / Level Adjustment (LA) / Average Party Level (APL). It really isn't hard, but people seem to freak out about it enough that it should be explained in small, easy to understand words.
Thin Hardcover setting books for OOP settings, all of them; Mystara, Dark Sun, Planescape, Spelljammer, Greyhawk, etc. About 194 pages, or whatever the around 200 page book that WotC puts out now. Each would come with a 4'x6' full color cloth map (ala the olde Dark Sun Revised and Expanded map, but larger). There would be no further book support until the next edition, but there would be web articles every other week with more content.
No more Favored Classes. Every race works like Humans, in that their highest level class doesn't count for determining XP penalties.
Find the guys who made Fallout. Hire other programmers to help them make a D&D game of the caliber of Fallout, but with updated graphics and sounds and whatnot. The game would still be turn based by default, but there could be a real-time option.
Also, what a previous poster said about killing the Drow. That one. But I'd go a different direction after that. I'd reintroduce them as LA +0 Elves, exactly as the PHB elves, but with different Racial Weapon Familiarities; also with white skin and hair. What kind of sense does it make to have an underground subrace with dark skin?
Point buy will be the standard method of generating ability scores, and 4d6 drop the lowest will be included in a sidebar.
Lots of useful stuff from the DMG that really shouldn't be there (Wealth by Level chart, Prestige Classes, etc) will be moved to the PHB.
Also included in this expanded PHB will be a Gazeteer for the Generic D&D world (I'm going to say Eberron, because everyone I've talked to IRL likes the setting), only about 20 pages or so, just enough to get people going.
Personally, I loathe the Forgotten Realms, but realize there are people out there who like it. I'd release all the remaining "Area" books (Dalelands, Western Heartlands, etc), then wait for the next edition to publish more. Continue the Novel line, and make more expanded content available online, as with the other settings mentioned earlier.
Make .pdf files available for all releases, including Novels and Adventures.
Regain the rights to the Wheel of Time RPG, publish a new version with d20 Modern style classes. All the big name classes would be either Starting Occupations (Wilder, Initiate, Woodsman, Algai'd'sis'wai, etc) or Advanced Classes (Asha'man, Maiden of the Spear, Aes Sedai, Wolfbrother, etc).
Publish a Magic: The Gathering RPG. Make it d20, but not directly compatable with D&D due to different magic systems and the like (much as Wheel of Time was).
That's all I can think of for now, I'll probably be back with more soon, though.
-TRRW