Have the basic setup of the mother and son trapped in a car and being terrorized by Cujo but instead of Cujo being an actual dog make him a guy in a dog costume (like the one Harry wore in Dumb & Dumber), that would add to the creep factor tenfold. I have this image in my head of Cujo leaning against the barn at night, smoking a cigarette and glaring at the mother and son. Oh and the title of the movie could be Nice Dog.