I hear there's a particular theme for costumes every year... just curious what it might be?
Yeah, I've never noticed a consistent theme as far as what the random cosplayers dress as. I've noticed TRENDS - lots of Deadpool every year, and I imagine there will be even more now that the movie is out; conversely, the number of 40K cosplayers dropped from a few to almost none after the first couple years I attended.
There's a theme for the convention itself, and a related theme for the dance, and I suppose some people like to chose their costumes to fit with either of those. But there's no official costume theme that I'm aware of.