I'm also not (particularly) interested in producing my own retro-clone / D&D book with house rules "baked in". It'd be a fun project, for sure, but I don't feel inspired to go down that route.
What I am interested in, however, is re-inventing the implied setting of the game. (This is for no big reason, by the way, just for fun.) As far as I see it, the flavour of the D&D world is communicated by several things:
- The classes and races available to PCs.
- The magic spells which exist.
- The monsters populating the world.
- The magic items which can be discovered.
The classes I'm perfectly happy with. At least fighters and magic-users are generic enough to fit in pretty much anywhere. Thieves and clerics would simply exist (as adventurers) or not depending on the desired flavour, I guess. If I were to come up with something really original which merited work on new classes, then I might consider doing a Monte (q.v. Arcana Unearthed) and re-inventing all the classes as well, but I don't imagine it coming to that, and I wouldn't intend to do that just for the sake of everything being "new".
PC races are a bit of a bigger deal. One could either go for full fantasy and say that no humans exist in the world, or a human-centric (or perhaps even human-exclusive) world. I'd be inclined to do the latter, but if desired it's not hard to come up with a playable race or two. It would be very important for these to be unique, not just reskinned elves & dwarves, in order to avoid the standard D&D tropes, as well as the inherited Tolkien tropes!
Spells are a big thing. Some of the PCs' most potent abilities are defined by the list of magic-user and/or cleric spells. Even something simple like removing the sleep spell, or raise dead, has a big impact on the game. Me being me, I'd completely re-write the entire spell list. Not a single "canonical" spell would exist in this hypothetical re-imagining of D&D. (I think it was my recent musings on an Unearthed Arcana only campaign which also partly inspired this idea.) I'd probably go with a simple spell list of maybe ten spells per level, maxing out at 5th level or so. I'd probably also want to do something to encourage spell research, as it's such a fascinating and under-used element of the game.
Monsters are also a huge deal in defining the flavour of the D&D world. Again, I think these would merit a complete re-write. That's a pretty big project, but I think even a list of 15 or 20 flavoursome creatures, plus the idea that humans (& kin) can provide a huge amount of monstrous / villainous entertainment, would go a long way. Personally I think I'd focus on dimension-hopping fey and demonic beings.
Magic items have, in my opinion, a slightly lesser impact than spells (perhaps because, in many campaigns, they're rarer), but they still do a lot to define the extents of what's possible for PCs. Personally I think I'd be inclined to strip them way back, probably polarising them into very low power items and extremely powerful artefacts. Any kind of "+1" (etc.) weapons / armour would be right out.
Now I'm not sure if I'll actually get around to working on any of this... like many such thoughts, it's a bit pie-in-the-sky. But I thought I'd put the idea out there, who knows -- it might gradually blossom.