If Pikachu were to be retyped electric/fairy, I doubt the anime would spend that much time dwelling on it.. XD
New companion: "WHAT?! Geez, Ash, I thought you said you & Pikachu have been travelling together since you left Kanto? Everyone knows that all Pikachu are electric/fairy..."
Eh, I'd rather see the electric rat learn a few fairy moves. It would experience some of the benefits of the new type without having additional weaknesses slapped on... which could be better for it, but of course that depends on the rest of fairy's battle properties.