Thinking about my last post, about not usually including enough magical locations on the map, and an idea struck me for a megadungeon-based campaign.
What if the world is pseudo-historical in all respects, EXCEPT for the presence of the megadungeon?
If you go on wilderness adventures, there will be primarily humans and normal animals encountered. Giant animals and monsters would be extremely rare outside.
But the Megadungeon, following Philotomy's idea of the Mythic Underworld, is the one place where all sorts of monsters, and corresponding treasures, could be found. That's why adventurers are drawn to it, and wish to plumb its depths.
If you're not in the megadungeon, you're in a pseudo-historical game world, so the big darn dungeon is where the action of the game takes place.