Having burnt out on RPG design, I'm now kind of hoping that I like Spirit of the Century enough to do Hyperborean Mice as a FATE game.
The two big things that SotC doesn't cover are Race and Magic.
So, some potential ways to handle them...
Races could come with Aspects like Small or Tough. It could even just be part of character creation... read the description of your species and pick 2 Aspects based on what sort of critter you are. The race itself could even be an Aspect, although then you might have to look up exactly what each race implied, later.
Magic would probably have to be partially or completely Stunts. Maybe a spell-casting Skill that's only available if you take the Mage stunt.
Each race could also provide, say, 2 predefined Aspects and a Stunt.
Jumping Mice get the Superior Leap stunt.
I might well need several Magic stunts to cover types of magic, plus a few special ones.
Or perhaps Magic should just be a skill... maybe even a double-cost skill with high default difficulties. If it takes up 2 skill slots, a PC couldn't have a Magic above +4. But I like the idea of having specialized types of mages...
Enchanters, Sorcerers, Thaumaturges, whatever. The big groupings are Physical Magic, Mental Magic and Spiritual Magic. Conjurers, Sorcerers and Thaumaturges, perhaps... with Enchanter being an additional Stunt that lets you enchant items with temporary or permanent spells.
Hm. Okay, let's use the Mysteries skill. For HM, there are magic-oriented stunts that let you cast spells. Casting spells inside of combat is difficult and requires blowing a Fate Point, but it gives you a lot of outside-of-combat options.
The basic Stunt gives you access to a broad category of "Small Magics"... these always fail resisted contests (unless you blow that Fate Point) but add a lot of utilitarian options. Hm. Perhaps, instead of just costing a Fate Point, they require you to trigger an Aspect to work? That would mean that you could pull one off using the "free" use of a newly created Aspect. Or everyone just gets a +2 to resist them, so a really skilled mage can still pull it off?
A second Stunt (that requires the first) gives you access to the High Sorceries of that category... more potent spells. It may also enable you to declare modifiers more easily or something. Folks might also not get the +2 bonus to resist, making it much deadlier.