With Halloween over, I’m guessing there won’t be much more activity on this thread (not for another year, anyway), but I had a few more costume ideas for the gang I wanted to share, including some that would only make sense if the gang were familiar with modern-day Halloween traditions:
Chomper would put on a cape, maybe dust his skin with something that would make him look paler, and go as a vampire. (He’s already got the protruding front teeth.

)
All Petrie would need to do is paint himself black and put on some false ears, and he could go as a bat.
I would expect Spike to dress up as something relatively simple: a ghost, perhaps. Alternatively, it might be appropriate if he dressed up as a dog, since his character was originally based on Don Bluth’s dog.
I can't really come up with anything for Ducky, but I think it would be funny if she chose a costume so frightening that it ended up scaring the heck out of the rest of the gang.

Ruby’s even harder, but given her penchant for scary stories, I’m thinking maybe Hidden Runner or something.