I know you're probably thinking, "That idiot! They're already planning to add custom skin editors!"
No, no, I mean an editor that you can use to make fully custom skins. Each body part could be represented by a box and the user can use tools to draw the character. When a limb is edited, the limb on the other side is mirrored to it.
Tools: Eraser, fill tool, line tool, design tool, a tool to make curves and specified shapes, a normal drawing tool the user can use to draw the character using their mouse, and a copy and paste tool. Skins could be added to a skin list on the official website and the user could copy and paste the skin's ID code to their skin editor to use it.