I'd guess there are quite a lot that go beyond traditional Firefly roles.
Just a list (not to say the GM would like these):
- Confederate sympathiser/terrorist
- Former Companion/serial killer
- Police detective from one of the Allied Empire (Alliance) planets. Perhaps he arrested the 'wrong person', they pulled some strings and framed him, and he ended up on the prison transport.
- Former Blue Sun computer techie who hacked the wrong system, found out the wrong thing, and made a life for themselves out on the rim. Not as a technie but as an information trader/person-tracker-downer (stalker?) who got caught and locked up.
- A whore whose client got killed and she was caught by his body, surrounded by illegal substances and had no memory of what had happened. (She was framed? She wasn't?)
- A fortune teller who is absolutely a fake, and who killed her patron after his future didn't turn out as she predicted, and she had to kill him in self-defense.