"It is a period of civil war..." The opening lines of "A New Hope's" crawl established the state of the galaxy and included the premise of "Rogue One" The first of the "Star Wars" spinoff films, called "Star Wars" stories, takes place before the events of Episode IV. It centers on the group of resistance fighters responsible for stealing the plans for the Death Star. No, they're not Bothans -- that's the Death Star II. According to the Episode IV intro, these Rebel spies took the weapon's plans during the battle marking the Rebellion's first victory against the Empire. Given the time period and context, it's not absurd to think characters we know from the original trilogy to make appearances.
Rumors have been making the rounds about the likes of Darth Vader having a role, as well as Boba Fett and Grand Moff Tarkin. However, the playing field is bigger, and it makes more sense for the film to include smaller characters. It's likely easier to incorporate someone like General Jan Dodonna who doesn't have a huge role in the original trilogy. In theory, fleshing out his story would be easier than it would be with someone like Vader. Let's address the rumors for "Rogue One" before considering the possible inclusion of other faces from the original trilogy.
It's being reported Darth Vader has a large role in the film and will be one of the primary antagonists. He's undoubtedly a force to be reckoned with, and the animated series "Star Wars Rebels" has established Vader's enforcer role within the Empire. The show takes place very close to "Rogue One" on the timeline (at "Star Wars" Celebration, Lucasfilm senior vice president of development Kiri Hart noted the animated series and film are happening in the same period and hinted that the overlap presents opportunities), and it features a Darth Vader concerned with hunting the remaining Jedi and Force users and fighting the Rebellion. "Rogue One" is not a film about Jedi, so I'm not sure what sort of interaction the group of resistance fighters would have with Vader, but it's certainly a possibility.
Grand Moff Tarkin
James Luceno's "Tarkin" established good ol' Wilhuff was in charge of the Death Star's construction. The book took place five years after "Revenge of the Sith," so Tarkin was arguably still overseeing the finishing touches on the battle station during "Rogue One." He was also tight with Emperor Palpatine. If any character from the classic trilogy were to have a role to play in the upcoming film, I'd put my credits on Tarkin.
A reveal from LEGO at the Nuremberg Toy Fair showed a ship looking suspiciously like Boba Fett's Slave I as part of their "Rogue One" collection. He's a fan-favorite character, and his role as a bounty hunter means he could show up anywhere in the galaxy working for anyone. Marvel's "Darth Vader" comic books have revealed Vader didn't first work with Fett until after "A New Hope." As one of Jabba's finest bounty hunters, Fett could cross paths with the resistance fighters; maybe they even work with him for the sake of getting the job done. If bounty hunters are in the picture, it means we could see cameos featuring the likes of Greedo, Dengar, Bossk, etc.
Casting for a younger Han Solo is happening for the second "Star Wars" story spinoff, and it's rumored the actor will have a cameo in "Rogue One." It could happen, but I wouldn't expect more than a cameo because Han doesn't really get involved with the Rebellion until the end of "A New Hope." He could easily appear for a second in a spaceport as a fun nod to introduce the actor, but it's unlikely Han would have an impact upon the story.
With rumors out of the way, let's consider who else could plausibly pop into the "Rogue One" story and why. Keep this in mind though: Since this will be the first spinoff, they have plenty of reasons not to lean too heavily into the existing pool of characters.
Jabba the Hutt
If Boba Fett's around, I'd expect a mention of Jabba the Hutt. I don't think we'll necessarily see the slimy slug because I don't see this film stopping by Tatooine, but he is one of the most powerful crime lords in the Outer Rim, if not the entire galaxy. He's on the Empire's radar and probably the Rebellion's, too.
No. Seriously. In "Star Wars: The Clone Wars," we saw Sy work as a spy for the Hutts. It's feasible to think her work would send her around the universe, and she'd be a fun cameo to include.
Bail Organa and Mon Mothma
The Rebel Alliance didn't come together in an organized way until the years before "A New Hope." Bail Organa and Mon Mothma were the architects of the group. We've seen Bail and his adopted daughter Leia in "Star Wars Rebels," so I would expect references to them at the least. Of the two, I think it's more probable Mon Mothma would have a part.
Rebel Alliance Leaders
"Rogue One" will focus on the everyday soldiers of the Rebellion; it wouldn't be surprising to see cameos from people we know as leaders from the original trilogy. This could include Jan Dodonna, Crix Madine, Carlist Rieekan, and Gial Ackbar. We could see them moving up the ranks or as rookies.
"Rogue One" will dive into an interesting yet undeveloped portion of the canonical timeline. Lucasfilm has the opportunity to use the plot to develop characters we already know while introducing new members of the Rebel Alliance. Since we've already seen them take actions to connect the dots between eras -- like how they're tying "Star Wars Rebels" with "The Force Awakens" by having a lightsaber design similar to Kylo's in the series -- I imagine we'll come across at least a couple familiar faces.
Directed by Gareth Edwards and starring Felicity Jones, Mads Mikkelsen, Alan Tudyk, Donnie Yen, Forest Whitaker, Diego Luna, Riz Ahmed and others, “Rogue One: A Star Wars Story” hits theaters on December 16.