[Gurthang]

Gollum would definitely be Santa. I don't know why, he just would be.

Aragorn would dress as a Jedi. Cuz Jedi's are cool.

Legolas would dress up as Chief from Halo. Shooting and all that.

Frodo would come as Donald Trump.

Gimli would be Mickey Mouse. ??

Pippin would forget about the party because he was drunk in anticipation.

Gothmog would come as himself, and win first in the scariest costume contest.

And finally: A Balrog would come, with wings on his back, and no one would be able to say if the wings were a part of him or his costume.