RPG Setting Musing:
The first day it happened, we call that the Severance. We don't know what caused it. Most storytellers will tell you though, it was a severance of us being the dominant species. We lost our technology, our civilization, our place in the world. It severed everything. Humans used to tower over the landscape, taller than dogs or cats. Now we scurry, just barely the same size as mice.
Now we are tiny, the world has become a desperate struggle day to day. Once loyal animals have become our most dangerous enemies: where we were severed from power, other animals were granted parts of our former dominance. Still far larger than us, we struggle to survive in new cities we've created. We are too far apart to be able to rebuild our nations, and we are too small to recreate our technology. As generations pass, the stories of our civilizations pass into dreams, just as Dogs, Cats, Crow, and others begin to create new nations of their own.
Severance, a generational post-apocalyptic RPG of humans struggling to survive the world they themselves created.
Humans in the setting lack magic or anything else. The major shift was shrinking to the size of mice. The main opposition is nature itself. As for animals, most domesticated large animals have 'uplifted' a bit. They aren't rocket scientists, but when they started talking and planning in ways humans didn't expect (cats that had developed weapons, for instance), these animals thought more about creating their own communities, and the tiny humans are just minor pests from a past those animals would rather forget.
Alternatively, while humans have no magic, perhaps these animals have. Who knows.
As an alt history angle, I'd imagine the first generation of the Severance to take place in 1985. That seems a safe bet that a good chunk of digital tech isn't around to be a easy work around for problems. Each game could be set in a different generation, however. A second or third generation game might have little to no understanding of their cultural past.
Lastly, I'd like some form of domestication to continue, but it carries over to smaller animals that seem to have remained the same in most instances. Ferrets, mice, ants and others. Flavor for location, as always.
Inspirations: A picture I found on google more or less started this (see at the top).
Other possible sources: The Redwall series, The secret of NIMH, Sandman (A Dream of a Thousand Cats specifically).
That's the nutshell of it. I might try to expand it a bit, but that seems enough for a random thought to me.