Super Hero Birthday Theme Party :)

So my 8 year old step son is having his birthday party this Saturday. We have decided to do a super hero/ comic book theme. He is wanting everyone to dress up as super hero's and come to the party :) yes, only my step son would come up with this idea haha. So i am purchasing him a Xman Wolverine costume at the only store i found that had costimes right now (in mid Dec lol) however the rest of us will not be pruchasing new costumes so any ideas of what we could be? We have invited about 8 little kids, who are all wearing their Halloween costumes (not all super hero's) and my husband will dress as the hulk (green paint on his face, white shirt, and purple pants [omg :)] ). My brother will dress as a Ninja turtle (since he has loved them since he was a small kid) but my SIL and I don't know what to dress as. A cousin that will come is going to be Cat woman (since she was a cat for Halloween) and will reuse the costume. I was thinking of getting deco that is just different characters (spider man, xman, bat man etc.)

Any idea's will help :) i know he will only be small and requesting this odd stuff now so we are going with it haha.