Start your engines for this, our fortieth mashup.
I’m going to use Greyhawk once more, as it’s the archetypical D&D setting. One of the things about Greyhawk is that there are plenty of stray dimensions lying around; there’s always an alternate Prime Material Plane available when you need one. The setup to this adventure is the archetypical greybeard wizard who sends the PCs across dimensions “to set a horrible situation to rights.”
They arrive in a Greyhawk in which kobolds are the dominant race, acting just like humans, and humans are the fodder for low-level adventurers.
Really! Castle Greylizard. The Spear Coast. Shamans instead of clerics. All that stuff. And you have humans who are bossed around by mean ogres and so on, and the humans are reviled and scorned for good reason. They simply aren’t that bright. People talk about the sage named Tucker who posited intelligent, clever humans — but nobody takes him seriously. The PCs are a huge aberration.
Dwarves are like orcs, I suppose. Elves… occupy the hobgoblin ecological slot? Gnomes are like goblins, and halflings are — I’m out of small humanoid races. Halflings are like goblins too. It doesn’t really matter which race corresponds to which, as long as humans are on the absolute bottom.
The big reveal is exactly the same as in the original movie. At the end of daring adventures, freeing slaves, and so forth, the PCs discover something — probably a record of one of their adventures back “home” — which makes it utterly clear that they are in the future and not another dimension at all. No going home, but at least they set their fellow humans free.