I recently read the whole of the recent Mouse Guard comic series, and it gave me some ideas for a offbeat game world for Iron Heroes...
"Nobody knows what happened to the Ancients, but everybody agrees they were big. The rats claim their ancestors brought the humans down for their dark god; others say the Ancients transcended on to some sort of Heaven. Most of what they have left is monolithic and mysterious, and many mice have raided their ancient stores of grain and things far stranger. Their former cities are home to more hostile beings, such as the savage rats and fearsome predators."
"We mice have become smarter and stronger than our forbears, no longer restricted to simple burrows or four legs. We are the only civilized race. While the clans of rats are cunning and the ants numerous in their hordes, we alone have learned to work with the metal and resin left behind by the Ancients to create something more. The fallen branches build our homes, and we find protection together. Though the predators that hunt us in the wild are nightmarish things, by banding together we can survive and grow..."
"It is a sad thing, then, that we still have so many violent disagreements..."
Iron Mice is a concept for a Iron Heroes game, replacing the usual human heroes with anthropomorphic mice. Though it would take place on Earth, that fact would be academic, since travel times would be greatly extended due to the size of the heroes involved. A medieval-era society of humans ("The Ancients") is presumed to have existed at some point, but plague, divine fiat, or similar disaster wiped them out, leaving only their ruins behind. Everything would be tweaked with mice as the baseline, so mice would be medium-size, rats would large, cats would be huge, and so on.
Most of the material in Iron Heroes would be available to them; the lack of magic in the world would mean the Shadow Born background and the Arcanist class would not be available. Armigers would be rare, and work with lighter forms of armor than the average human member of such a class, as a slow mouse would most often be a dead mouse.
Unlike normal humans, mice would have some additional abilities as a race - a 40' (or '40 seeds' by mouse reckoning) move speed, low-light vision, scent, and a 1d4 bite attack and two 1d4 claw attacks as natural weapons. Other than that, they would play like normal Iron Heroes characters. As for other races and monsters... well, perhaps that'll come another day.