I just have this like image of Sam and Dean walking out of their hotel room to find the Impala missing and all their guns and stuff in a pile next to an unconscious/sleeping girl.
It would be awesome. Maybe she doesn't know how to talk and she's kind-of in love with Dean. A modern day Snow White built to take on an army. It would be awesome!
I tried to pull my thoughts together and write it, but alas I didn't succeed, someone else should totally try it out.