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.

There are a number of interesting things about Rats such as their need to chew (as their teeth continue to grow). They are constantly gnawing on things and can chew through a wide variety of things such as bone, wood, cinder blocks, lead pipes (not steel plating though), etc. Maybe you could have a Bite ability as a melee attack, but also as a way to dig through certain terrain/obsticals? There's also their keen sense of smell, which can so sensative as to detect Landmines and even diagnose tuberculosis. They can also purportedly carry and transmit a number of diseases, such as bubonic plague, lassa fever, leptospirosis, Hantavirus, etc.

If your going for some anthropomorphising, you could also try mining some other works for idea's, such as The Secret of Nim, The Redwall Novels, The Tale of Despereaux, the rats in Dishonored, etc.

The bite idea is interesting, but I'm not sure it'd look good in first person. I'll keep it in mind, though - there's probably some way to make it not feel weird.

Diagnose tuberculosis as a gameplay mechanic? xD Well, I'm sure a Twilight Princess-esque smell-o-vision could work as a scan visor-style way to discover stuff you can interact with and hidden secrets.

I can see carrying diseases work as a pretty good passive ability, as well... basically, enemies you melee successfully would get poisoned and lose health over time and/or lose stats.

You simply have the player betray one of his friends, so he will be a rat :)