Well, a campaign setting! It is for a campaign that just started with a group I played with in 2008-15ish, and based loosely on the homebrew I used then. Or are you asking for more specifics about theme and atmosphere and such? The short version: A post-(magical) apocalyptic setting which draws from Sword & Sorcery (Howard et al), Earthdawn, Middle-earth, and other sources. What it doesn't draw from is the more anthropomorphic and video-game inspired ideas of the last 20 years or so.
Anyhow, this brings me back to my underlying (but unspoken) point above: I see the vast and varied corpus of D&D ideas as a toolbox, which each individual DM draws from to create a campaign setting. What a DM picks and chooses is entirely up to them, and what sort of setting they want to create. If it is a more co-creative setting, then players are part of that process.
WotC can model this in different ways through their official settings - which is what they're already doing. Each setting has different thematic qualities, but they range from "anything goes" to "strongly thematic." This, I think, can inspire creativity in DMs, if they recognize the toolbox idea. "Ooh, Theros is cool - I want to design something similar, but use Mesoamerica as inspiration instead of Greece." Or, "I love Dark Sun, but it is a bit too dark for me...how could I adapt it to being part of a larger setting, perhaps as an isolated continent ?" Or, "I like the anything goes approach of the Forgotten Realms, but could I build something similar but a little more cohesive, in which everything doesn't seem to haphazard?" Etc.