I love Thresh! He's so much fun to play >.<
I am actually working on a cosplay of him too.
Here's what I have so far:
Face/mask thing> http://frekki.wordpress.com/2013/04/07/thresh-mask/
Can't wait to see what other Thresh cosplayers come up with
I think incorporating lights would be a nice touch, but its not something I know much about, although I think it would work out way better than glow in the dark anything.