A family made up entirely of ghosts could also be an unusual concept. Their story could revolve around their after - life in the Sims 3 world. Maybe they haunt a particular place and try to communicate with the living Sims. They could have their own ghostly traditions, like having a spectral party every Halloween. And they could try to influence the lives of the living Sims in positive or negative ways, depending on their personalities.
How about a family of aliens and Sims? The aliens could bring their advanced technology to the family, while the Sims introduce them to normal human (or rather, Sim) life. Their story could involve the aliens trying to fit in, like going to school or getting a job, and the Sims learning about the alien culture. It would be really interesting to see how they interact and what kind of family traditions they develop together.
A family of vampires and werewolves could be an unusual combination. The story could be about how they try to co - exist despite their natural differences. The vampires could be the more refined and cultured ones, while the werewolves are more in tune with nature. There would be a lot of drama as they deal with their different needs, like the vampires needing blood and the werewolves needing to run free during the full moon.