I love the Mediveal Idea. Knights and horses and dragons wicked.

Vegetables

Video Games
History
Bums (opposite of Uppers)
Goverment
The list of ideas could be endless.

How about the Innocent Bystander clan. All those ordinary citizens of Clint City who have to deal with the superpowered/magical/technologically advanced clans rampaging about the city and inconveniencing their lives on a regular basis. Their clan ability can be similar to Rescue's Support ability...only in this instance the ability will represent the regular citizens having formed an angry mob to finally deal with the situation. (sorry, couldn't resist being silly...)

The new clan should be called The Union because all the other clans keep destroying the city and the construction workers the building engeneers, garbage collectors and all the clint city workers should get together to try and stop the other clans from destroying the city. The Leader should be called Jackhamer a real strong construction worker with yellow hard hat and a jackhammer and his partner a female building engeneer with some blue prints in hers arms... I think that type of clan is need to try and stop the other clans from destroying the city and their bonus should be B.O.A, bonus =opp. ability. but all this is just my wishful thinking.....

Food people

like mersh the marmellow

There could be a fantasy like clan, walking tree, elves, halflings etc.
Though I would be particularly fond of a homeless/alcoholic clan.

Wat about trade forces?

Egyptions

all the gods, angry pyramids , mummies i think that would be pretty good

DOTA characters.. like mogul khan, earthshaker & sven..