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).
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