I have read multiple stories about the return of Darth Vader. Here is my vision on how this can happen:
During the clone wars Anakin was captured and was secretly cloned. This would open up multiple possibilites for the return of the character. Here are two:
1. The clone returns to the main story complete with all of the memories of Anakin Skywalker. The Empire then hides Anakin on some remote planet. After the fall of the empire, Anakin manages to escape and goes into seclusion. Years later he makes the decsion to return using an assumed identity.
2. The clone doesn't have the power of the force and is eventually discarded by the people who cloned him. He ends up on a remote planet and lives out his life unknowing of his identity . When Darth Vader dies, he aquires the power of the force and the memories of Anakin Skywalker/Darth Vader.
Each possibility can lead to the Return of Darth Vader. I can't wait until he meets up with Luke and says, "Luke I am your father".