I've played around with an idea about a game that claims to be a serious rat life simulator where you evade predators and eat cheese and fruit to get stronger... but pretty soon into the game you start picking up (rat-sized) guns and start exploding things. Now, that in itself would just be a gimmick that'd surprise people once, but then I started thinking about ways rats differ from humans. I know rats can hold things in their front paws uncannily nimbly (I've had pet rats before and gotten to study them in great detail), so you could kinda argue they'd be able to lug guns around, but since they're also quadrupedal and stuff, which humans aren't...
Basically, I'm looking for ideas for gameplay mechanics that would apply to rats but not to humans, and figured it would be an interesting discussion topic. I've only gotten one so far, that you run a lot faster when in quadrupedal mode (unequip any weapons so your front paws get free) than when running around on your hind legs, but I'm not sure it's enough for a full game experience.