Dean/Impala. BUT! It sort of makes sense! Maybe the car got hit by some magic, maybe it's always secretly been a chick, maybe Dean's love and devotion to it magically turned it into a girl, but somehow his beautiful car has become a real human person. (We all know she's the only love he's ever been faithful to.) She can shift to and from the car, or is now just a girl, or whatever. Maybe it's even a guy and has resented being called Dean's baby all these years.
So. I'm open to PBs, plot, etc. This is crack, okay? I know that. But it could be fun crack with a side of adventure and awesome?
Also, as a side note, would love to do a Sam & Dean PSL, too. WINCHESTERS. FIGHTING THE GOOD FIGHT AND WHATNOT.