Like, height, weight, dominance, colouring, clothing (or lack of it), etc.
OK, please, send me a PM with any specific requirements you have for their design.
Also, do you want it to be sprite-like or more freestyle? Spritesque style will be more complicated to develop, but much easier to use, while freestyle will be easy to develop, but much harder to use.
OK, I'll start designing the concept arts then.