Alot of the shots you took seem to be simply on the spot! Do NOT hesitate ask the cosplayer to pose a certain way because they WANT their photos to look good. They will not get annoyed at you if you ask. Like the SAO cosplayer, she would look much better if she posed in a different way. Same for the hammer cosplayer.
Some of your shots seem to have lighting issues too. Like shadows on the face and etc. If you're taking shots look around you and see where the greatest light is coming from. Make sure they are facing towards the light so there won't be as many shadows on their face! And if you find that you have shadows in the picture try to fix it in post-processing.
It might just be my opinion but sometimes looking at the camera doesn't really show the correct mood of the cosplay so you might want to tell them to look that way or this way sometimes too.