The very first thing that leapt to mind.
• Premise – What’s this about? What do characters do in this setting?
PCs are teams of mice working for the Free City of Leadhaven. Emerging from the ruins of a post-holocaust disaster, they work to set the world right again.
• Missions – What kind of missions do the characters go on?
Their job is to go out into the blasted wastelands of the world and survey the countryside, make contact with friendly survivors, drive off enemies and recover lost technology.
• Conflicts – Does the setting require new conflicts? How do they work?
I believe conflicts work the same way.
• Weapons – What are the weapons for the conflicts?
Obviously high-tech wepaons (guns, lasers, missile launchers) can be found/scavenged and provide significant bonuses over low-tech weapons. Perhaps they give a bonus and add to your starting disposition (“Good, bad, I’m the mouse with the gun.”).
• Overarching Conflict – If your setting isn’t about outdoor survivalism, what’s the overarching conflict? What’s the implacable force?
Radiation. There’s plenty of outdoor survivalism, but it gets completely weirded up with radiation-related complications that make things worse.
• Territories – What are the Territories for this setting?
Territories are less about mouse settlements and more about what’s in the general area. Maybe it’s mice, maybe it’s mutant beetles, or a dead zone. Each territory provides a fixed source of potential conflict with Leadhaven.
• Denizens – What creatures or NPCs populate the setting?
same as in the base book, plus messed-up mutated versions of everything.
• Skills – Your setting’s going to need new skills!
Skills stay roughly the same, however there are one or two skills/wises related to the analysis and use of pre-disaster technology. Those skills are available as per the normal rules for pure strain mice (they may even get some for free), at a severe cost for mutant mice, and unavailable to mutated animals (i.e. everything else).
• Abilities – Come up with new obstacles for Circles and Resources for your game.
• Traits – What are the traits for your setting? These are easy to come up with!
Most Traits are really mutations. You can gain new mutations via radiation exposure (be sure to take radiation-wise!). Pure strain mice are very likely to get horrible mutations, mutated animals are likely to get something great and mutant mice take their chances.
• Recruitment – This is the biggie. Recreate the Recruitment section so that it fits your setting! Rename the steps, but keep the same number of steps and the same point totals.
The big addition is choosing between pure strain mouse, mutant mouse or mutated animal. Mutated animals can only come from the list of animals that are one category larger than mice or below (no mutant moose).
It’s a little unfinished, but it’s a start.