Good awareness of the general shot composition (uncluttered backgrounds, good exposure, etc.). Since you have those elements under control, the next step is to help your subjects relax and pose more. In many of these shots they look stiff. A pro model knows how to mix things up a bit, your average cosplayer less so. Compliment them and have fun!
As another small suggestion, take the time to identify the characters in your photo descriptions. It will help people find your photos and makes the shots more than a hallway moment.