Well, a lot of people have said that it might be a lizard or a frog, (I was hoping for something mammalian and adorable, but oh well,) and Platonic said that unveiling their characters now would reveal too much of the gameplay, so....
If it's a frog, we'll undoubtedly see use of that tongue, and that long-legged jumping-prowess. And maybe the croaker, too. Although this might also mean an abundance of water levels. But Rare games always had a lot of water in their games anyway.
If it's a chameleon that'd be super cool, but I'm not sure what they could actually do with the color mechanic. All sorts, I guess. We'd also probably see use of its tongue and tail.
Also I suppose it could walk up walls if it was a gecko, but a freaking BEAR was shown to be able to do that just fine.
"Tiny creatures far below, which of you'll be first to go?"