Hayden Christensen as Anakin Skywalker was originally supposed to be in 'Star Wars: The Force Awakens'
Not long after director J.J. Abrams revealed that McGregor came in and lent his voice for Rey's dream sequence in the new movie, it turns out the film's concept artist, Iain McCaig, had Hayden Christensen, who played Anakin Skywalker in two of the prequels, in mind when sketching one particular scene, which would've involved the ghost of Anakin.
According to ScreenCrush, this is how McCaig came up with the idea, as featured in the "Art of 'Star Wars: The Force Awakens'" book:
If you've seen the movie, you know that Anakin (as Christensen or Darth Vader) doesn't appear in the movie, though Darth Vader's mask does make a cameo.
Which makes you wonder if this might be an option in future films.