Wow, this sounds awesome! Good idea! I should probably throw out the idea that alot of games have a rock/paper/scissors based gameplay, aka flying/ranged/melee, fire/water/grass, or axe/sword/spear. So an idea would be earthpony/unicorn/pegasas.
This is kinda close to the whole "Face" thing, but have you wondered if alicorn Twilight will have a longer horn and larger wings then normal ponies? It doesn't really seem that way, not yet though.