Taking an interesting shot of a cosplayer showing their full body is actually one of the most difficult images to achieve. Snapshots are a dime a dozen, and often a snapshot is a full body shot with large depth of field, showing crowds of people in the background making the subject not stand out much. In my experience, interesting full body shots are best taken with a nice large wide angle lens in the 10-20mm range. The subject should be in the foreground and the background should be vast open space with limited objects that would detract from your cosplayer. This is partly due to the difficulty in producing nice bokeh with a short focal length and most likely small aperture due to how most wide angle lenses are designed.
Taken with a 14mm rectilinear lens.
Another approach is to fill the frame with your subject from end-to-end and not much else. This is best done by rotating your camera so that the subject’s head and feet fit in their respective corners of the frame. Try having the subject slant the upper half of their body downward towards you so it gives the feeling that they are coming toward the camera (take note that wide angle lenses have less discernible depth of field in front than in back generally). Other options would be to have them sitting down or in a pose that makes them less stretched out to give you some feeling of depth while still having them completely in the frame.
Leaning forward wide angle group shot.
You can try incorporating the background into your image making use of large depth of field and creating a full package of interest in the frame. If you are on a private shoot, find a location that fits well with the person’s costume. For example, if they are in a dress that looks like it is part of the Victorian era (called Lolita fashion at Anime conventions), find a piece of furniture that has a historic look to it. You could even find a wrought iron park bench that will allow you to maximize bokeh if there is nothing behind the bench. If you have multiple people from the same group available, take them outside or to an open space, positioning them at different distances from the camera thereby making the background part of the interest as the other person will be relevant to overall concept. It would almost be like creating a short story in a single frame. The person in the foreground looks off into the distance as the one in the background, partly blurred, is walking toward the foreground. Applying action to photos can greatly increase their interest level.