One of the things that I have always, ever since I first started playing, wanted to do with D&D was to redo the races and classes. Not because I was particularly dissatisfied with them; I just think it would be a fun project. This is one of the reasons why OD&D and its derivatives appeal to me. The simplicity of the rules, and particular the open-ended way that racial and class abilities can be detailed, makes this kind of project a lot more realistic.
Note that I've never actually gotten around to doing anything like this. I've made a few half-hearted notes, here and there. But mostly, it's just been on my "someday" list. For years.
Partly this is because I've run mostly various flavors of 3.5 D&D, and while that's the DMG that gave me the idea in the first place, it'd still be a bear to do. But mostly it's that I never had much of an idea of what I would do if I redid the class lists. My vague thought was that this would be part of some serious setting project, but that's never quite gotten done, either.
Lately, though, I've been thinking that this might make an interesting side-project for my megadungeon game. (Which I am still working on; my hope is to have it ready for at least a few hours of play by next weekend.) I'm already turning over ideas for replacing dwarves with some kind of "mechanical men," sentient, humanoid versions of the caretaker robots that occasional escape the dungeon, or that the dungeon sends out for its own mysterious purposes; they would obviously have certain advanced knowledge of the various dangers of the facility.
The nice thing about doing it this way is that I know I won't be running the megadungeon as a continuing campaign for a while. So I have a fair amount of room to mess around with things, even if I'm running it every so often. The important stuff is what's going on inside the dungeon, anyway. So far, I haven't given much thought to the larger world outside the dungeon, but my plan is to generate the world outwards from it as it pleases me. I don't have to worry for now about detailing even the region around the dungeon, because any "test drives" over the next few months can easily get away with being dungeon-exclusive.