Privateer Princess we needed humans, aliens with telekinetic hair, and Offspring - hybrid human-alien crosses with huge sideways elf-ears (or just elf-ears in the Japanese style.)
If it’s a fantasy/sci-fi story like ours you’ll need to consider who is the dominant species in the world and how many of the less dominant species there are. Even though it’s more exciting and fun to draw the funky, cool-looking characters in your comic (not to mention your hero), drawing ONLY them takes the thrill out of them for the reader. What I mean is that without having at least as many “normal” characters in your story as extraordinary ones, you'll lose the feeling that your “super-cool” characters are in any way special. (This is not the case if your entire world's population is alien (or definitely not human), then ordinary humans become extraordinary!)
As in any art, contrast is the key. So don’t forget to have lots of “ordinary” people in your story alongside your really extraordinary ones. Even if your story isn’t about aliens, or vampires, or fairies, it’s important to show your main characters (who are by definition special) surrounded by plenty of regular guys and gals to point out how unusual they really are.
Next up: Part 4: What kind of story do you want to tell?