It has been a rumour for a while, but now it has been confirmed that Darth Vader will be appearing in Star Wars: Rogue One. The film will be coming out this December and is set between episode III and IV about the stealing of the Death Star plans. (So it's not really THAT much of a surprise that Vader will appear...)
Of course, the biggest takeaway here is the presence of Darth Vader; the return of the infamous Sith Lord has been rumoured for months, and now that his appearance is confirmed there's a good chance he might show up in Rogue One's next trailer. As for what Vader will be up to, EW's article states that Mendelsohn's character will be hoping to "avoid the wrath" of the Emperor's "black-masked enforcer", and we all know what happens to those who get on Anakin's bad side...