This is something I've never seen being talked about. What would've happened if Anakin was left on Mustafar in episode 3? What if Palpatine never came for him? Well, the straight forward and most simple answer is that Anakin will eventually burn to death and Palpatine won't have him as an apprentice. Palpatine might start looking for a new apprentice after the end of Revenge of the Sith. When Luke eventually becomes a Jedi, Palpatine would come after him. Obi-wan and Yoda would try to intervene as they would see a similarity between what's happening to Luke and what happened to Anakin. They would stop Palpatine from trying to get Luke to the dark side and Palpatine will still have no apprentice.