OK, half formed idea to follow, but I think it has potential.
Imagine a D&D campaign starting up. Sandbox style, possibly with a tent-pole adventuring location (mega-dungeon) near the starting home base.
The sandbox is full of low fantasy swords & sorcery style locations/NPCs/adventure hooks. Duplicitous thieves' guilds. Ruins full of loot, guarded by unsavory things from a past age. What passes for civilization in these parts are wretched hives of scum and villainy.
But there are rumors, legends, travelers' tales about what lies just beyond the edge of the map (or far beyond it). Lands of evil magical overlords, legendary dragons and their hoards of gold, kingdoms of light and darkness engaged in eternal warfare.
The important point is that this setting is a sandbox. Players would be free to stay in the starting location their entire careers, looting ruins for treasure, then carousing away the winnings on harlots and black lotus powder, then skipping town just before the enforcers from the guild come to collect on the debt to try their luck in another expedition to kill things and take their stuff.
OR, they could listen to the rumors of the far off lands, the great evils that need fighting, the legendary monsters that could be slain if only the right legendary weapons were to be located, and set off on a Campbellian hero-journey.
Would there be an audience for a product like this? A setting book that gives a DM the tools to run a Lankhmar/Conan-esque S&S game and also an Tolkien/Eddings/Jordan-esque epic high fantasy game, in the same game world? Without being 400 pages long?
I think the beauty of the idea is that a set-up like this would allow for occasional crossovers. The epic questers might take a break and do a bit of dungeon delving. Or the cutthroat treasure hunters might decide to go after one of those magic weapons to make dungeon delving easier.
So, any settings like this? The Known World/Mystara was sorta like that after the fact, or it was at least in the way we played it back in the day. It was set up to be a place modules could easily be slotted into, but eventually (thanks to the epic scope of the Companion/Masters/Immortals sets) became the home of far-flung epic adventures.
More importantly, would people find such a product useful for their games?