DuracellEnergizer said:
Assuming the "Dark Luke" rumour is true, it might not be the real Luke; it could be a shape-changer who's assumed his form.
Or Luke could be possessed by the Force ghost of Palpatine, though that doesn't explain why he would be holding Vader's helmet reverently.
If they try to tie this strongly to Star Wars Rebels, we could be seeing a shapeshifter who was one of the inquisitors.