Both the timing and the various settings of Rogue One make the possibility of a brief cameo from Luke Skywalker closer to reality than you might think.
Jyn Erso’s Death-Star-Plan-stealing errand is actually taking place in the same neighborhood of the galaxy where Luke Skywalker is bouncing around. It’s entirely possible, for example, that Erso will find herself recruiting on Tatooine before her mission kicks off. Luke’s home is right around the corner from a top-notch hive of scum and villainy, which could be a great place to pick up some ne’er do wells with a death wish. So, with the possibility of Tatooine being briefly glimpsed, we’ve got one way for Luke to stroll by.
It’s even more probable, given the prominent connection between Rogue One and A New Hope, that the survivors from Rogue One’s suicide mission might bump into Luke in a different setting: the Rebel Base on Yavin IV. Assuming there’s some kind of epilogue in the movie, maybe Jyn will see Luke as the Millennium Falcon is touching down in the jungle, or even give him a nod as he’s running to the cockpit of his X-Wing before blowing up the Death Star.
This young-Luke cameo could be convincing, too. Thanks to Robert Downey Jr.’s recent youthful appearance in Captain America: Civil War, we know the tech that would allow for Mark Hamill to play his younger self is totally workable. Plus, we recently saw Mark Hamill shave his beard off on Twitter. Maybe the smooth face was for some de-aging motion capture?
And if we’re very lucky, Rogue One could give every Star Wars fan their greatest dream: glimpsing Luke in the background of Tatooine’s Tosche station haggling with a goofy alien over the price of some power converters.