I would recomend picking out some sort of a color scheme. For example: have one side all characters with warm colors and the other side all cool colors, or maybe the top half has all characters with a main color that contrasts with the main color of all the characters on the bottom half. Do a google search for color theory and you'll seriously be on your way to coming up with designs that are much more pleasing to the eye. Whether he meant it or not, this is really what makes SNAAAKE's side art and cp images look so nice.