I would like to see a campaign world that is not a finished, complete description, and never will be. Most of all, I want no future support. No endless succession of cataclysms or times of trouble, no shake up in the pantheon, no reforming of land masses. I don't want any revisions, any additions to the incomplete timeline, and no novels detailing the backstory of its sketchy NPCs. This would be a one-shot product, and would be marketed as such.
I want a framework of a campaign world, with incomplete descriptions of cultures, kingdoms, deities, factions, threats, and NPCs. The maps should feature a lot of blank spaces, not just with a missing continent somewhere over the sea, but in between more developed areas.
It should come incomplete with unfinished adventures, and even the artwork could feature characters and settings that are partially finished, with half of the composition as a sketch. There should be lots of adventure hooks, a few unfinished adventures, a timeline featuring many "dark ages" where the historical record has been lost, and not every type of monster or race should be confirmed to be present in the world. There should be several versions of the cosmology, all of them naturally incomplete descriptions, and any one (or all of them) the true one.
This incomplete world should be in a boxed set, loaded with maps and books for both players and DMs. The DM book should give some guidelines on how to grow the campaign world around the player characters with suggestions on possible plots. The player book should describe only what a lower level player character might know. There should be large player and DM maps, the former very incomplete and probably inaccurate, the latter merely incomplete.
I want the DM without a huge amount of time to see this as a useful bootstrap for starting a campaign of their own making, with plenty of space to add entire cultures, kingdoms, and conflicts yet enough material that they don't feel the burden of doing all the foundation and heavy lifting. I want a world that sparks the imagination but doesn't smother the flames with overwhelming detail, one that a DM can pick up, learn everything there is to know about it in an evening or two, and launch a campaign the next day that feels like a complete world to the players yet is undeveloped just over the horizon. As soon as that first session comes to an end, the DM should be motivated and ready to start fleshing out the blank spots, building their world tailored to the player characters' adventures as the campaign develops.
In short, I want a campaign kit, not a finished world. And I don't want anyone to build it up for me with future releases.