To me, shape language has a lot to do with visual weight and the silhouette. Does the characters have more mass in places that give the character more powerful traits and personality that matches the rest of the design?
That is how I would start out, just doodle on top of a person at the right perspective, and also think about what size they are going to be in game, and exaggerate their features and differences, the lower right tiny images is about what size its going to be in game. Also look at starcraft and other rpgs and see what they do to show differences, and see what characters are hard to tell apart.