Getting high quality interestingly framed images in constrained areas is a big aspect of convention photography. If you happen to run into a great cosplayer and ask for a photo, you just need to work with whatever the area is like. If it is a small or busy room with crowds of people, you can use your photography abilities to try and isolate the person or group to make them stand out as the subject of your image.
Angled away from the dealer room tables.
Let’s say you are in the dealers room. You see a wonderful cosplayer browsing DVDs at one of the tables and don’t want to miss the chance of getting their photo. So you ask for a photo, but now you need to deal with the crowds and limited space. Shooting their photo with the dealer’s table in the background does not give you much room for nice background blur. Try having them move out about a foot from the table. Get yourself parallel to the cosplayer and the table. Now you can use the hallway or span of the room to your advantage since there tends to be at least a few feet of empty space in the pathway between sections of dealer tables. You can crouch down and take a photo of the cosplayer so that you get a lot of the empty ceiling in the frame as a background. You can also angle the camera halfway between landscape and portrait such that you fill most of the frame with the cosplayer to avoid background busyness.
There are many ways to get an interesting shot with limited space besides those simple ideas like using a wide angle lens, taking a portrait instead of a full body shot, and asking the cosplayer if they would like to take a photo in a better area. Be a king or queen of improvisational photography!