If the guys at Schmoes Know are to be believed, you'll want to stick around after the credits of J.J. Abrams' Star Wars: The Force Awakens. Rumor has it we might see a tease and a small bit of footage from Gareth Edwards' Rogue One: A Star Wars Story at that point, though no more details are currently available and they stress that, despite the trustworthiness of their source, this should be considered a rumor instead of fact at this stage in the game.
The interesting part of this, if it's true, is that we know Rogue One takes place before the events of A New Hope, as it involves the stealing of the plans for The Death Star. And since The Force Awakens takes place thirty years after Return of the Jedi, that means the film's post-credits footage would be teasing events that took place more than three decades beforehand. That's sort of strange in a world in which Marvel has popularized post-credits scenes and they generally look ahead in the cinematic universe's timeline, not backwards.
I doubt the validity of this story because I feel like Lucasfilm is going out of its way to keep the new trilogy separate from the "A Star Wars Story" side films, so teasing one of those instead of, say, Rian Johnson's Episode VIII (which is reportedly shooting this month) doesn't really make a lot of sense to me. We'll see how it all plays out when The Force Awakens arrives in theaters on December 18th.