Every single time this comes up I've suggested the same 2 clans, Bikers (Many characters from Biker Mice from Mars could be used) And Mole people/Sewer people (Live in a city underneath Clint City and want revenge for all the polution).
There are bikers in the Game such as Jeyn "Beneath her violent and aggressive biker exterior....." and Katsuhkay "takes the lead of her biker crew and spreads terror in the streets of Clint City.".I wonder who re her biker crew. The Mole man as AiR-GeaR-Ikki said could be a part of Skeelz
HOws about a combinatoin clan? A clan that only unlocks once you have unlocked all the characters in a certain clan like, if you unlock all the Jungo, you get the sub-group insects, or if you unlock all the piranas you get the sub-group Aquanis. These clans will be special in that they activate the bonus of their previous clan but the new sub-clan cannot ever go CR (as a lot of times you will need some CRs to get into these clans. It would be awesome