I would have liked for a film with a suited Vader hunting down the Jedi. It would have had to have remained mysterious who he was. Nobody should know it was Anakin everyone should assume he died in the Clone Wars, and Obi Wan should think he killed him on Mustafar.
Not sure if this should have been in Revenge of the Sith or a film in between episodes III and IV. Maybe an episode 3.5