Brainstorm: A game where you transform into an animal at times, and when you are an animal you start to think like one (ie. you can't "talk" to humans, you start to want to mate / eat / hunt prey / play). This may be an animal such as a dog, or a 'were' animal. Invent an interesting mechanism that also explores the theme of what it is to be human.

But humans are animals. Most people want to mate/eat/play...

Yes, but what is it to be a Crocodile, a T-Rex, an emu or an Orangutan?

I'd say, find out about these animals on the internet or something. Read some kind of articles by some kind of scientists about the specific animal. Every animal will be different.

Humans in a nutshell: they only think about themselves. It's kinda hard to explain, but that's everything you need to know.

Some examples of this:
- Various sins (greed, lust, etc.)
- People improve themselves to improve the image they relay to others.
- People do things for others to either improve the image they relay to others, or to make themselves feel better.

And so forth...

The same can be said for all creatures, regardless of what they are (thanks to BorisTSR for pointing this one out).

Humans and animals have a lot of things in common. the main difference is we have reasoning and think a little more in advance. we still, however, have the same basic instincts.

i don't think you can invent a mechanism as you want. you put a human behind the controls of an animal, and they will still think like a human. if you wanted to do something like this, it'd be hard to make it interactive.