One big rumor (and potentially huge Rogue One spoiler) that has been floating around for a while is that Darth Vader may be making an appearance in the first of the spin-off movies.
According to Making Star Wars, a 'Darth Vader Dream Team' has been assembled to ensure that the Dark Lord of the Sith's appearance in Rogue One is authentic. Observant fans will have noticed that Darth Vader's armour varies slightly between the Star Wars movies, and the time period of Rogue One means that the Darth Vader costume shown on screen will need to match the original suit seen in 1977, with one of the most noticeable differences being the reddish tint to the eyes of Vader's helmet.
Brian Muir, The creator of the original Darth Vader helmet already announced on Twitter that he was involved in Rogue One in November 2015, and although he has made other creations for Star Wars, it's certainly not a stretch to imagine his involvement was necessary for a perfectly recreated Darth Vader. Additionally, rumor has it that James Earl Jones himself will be reprising his role as voice actor for the film.
Seeing Darth Vader in Rogue One would certainly make sense, as the film reportedly details the Rebel mission to steal the plans to the first Death Star, and we already know from A New Hope that Darth Vader was in pursuit of these stolen plans.