To improve? Let's see. Hmm, some characters are just out of place. @_@ You have some lineless characters, then some with outlines. Anime, semirealism and realism all mixed together. x_x And arranging the characters in a planned out way will be better. :0 Maybe strive for a balance in the direction each character is facing. Mm, you can try to make like one of those anime posters where everyone fans out from the central point overlapping each other and stuff. o_o Like kingdom hearts or something. x_x As for the background, you can also introduce elements into the far and mid ground to better "place" the characters in the scene, well, for digital art anyways.