I like both themes, but I feel as though Harry Potter would translate better to a pinball machine. With the Back to the Future theme I like the general idea of time travel and getting a playfield counter up to 88mph; kind of like the reactor in Total Nuclear Annihilation. It would also need to have Christopher Lloyd doing some custom callouts as Doc Brown. Beyond that, I don't think the plots of the movies work all that well, but I'm sure someone could prove me wrong.

With the Harry Potter theme, there is just so much material. Start out with the Sorting Hat announcing your house: Gryffindor, Hufflepuff, Ravenclaw, or Slytherin. Travel to Diagon Alley for supplies before making your way to Hogwarts. Pass though Platform 9 3/4 and board the Hogwarts Express. Watch out for the Whomping Willow. Complete your classes: Care of Magical Creatures, Charms, Defense Against the Dark Arts, Herbology, Potions, and Transfiguration to progress from a Level 1 - Level 7. You earn an Extra Ball by completing your O.W.L. in Level 5. Compete with the other houses for the House Cup, "10 points Gryffindor". Compete in the Triwizard Tournament (multiball). Play matches of Quidditch (multiball) where you are trying to score goals and capture the Golden Snitch. Save a lost ball with the Time Turner. Finally, I would steal from Champion Pub / Deadpool and have "battle mode" where you are dueling against other wizards, leading up to your battle with Lord Voldemort. There is just so much that translates perfectly to a 4-player game of pinball.