Solo: A Star Wars Story will undoubtedly feature plenty of connections to previous films in the franchise, but hopefully Ron Howard and company showed some restraint in regards to cameos. The film is a prequel covering Han Solo's youth from ages 18 to 24 as he looks to make a name for himself in the criminal underworld. Along the way, he meets his lifelong pal Chewbacca, encounters Lando Calrissian, and wins the Millennium Falcon. For a while, Solo has been described as an origin story for the classic character, establishing how he became the cynical smuggler Luke Skywalker and Obi-Wan Kenobi meet at the cantina.
The modern Star Wars films are chock full of Easter eggs for people to spot, and in some cases, viewers would argue Lucasfilm has gone too far in a few places. The studio's last anthology film, Rogue One, featured criticized appearances by A New Hope's Dr. Evazan and Ponda Baba (the cantina patrons that attack Luke) and R2-D2 and C-3PO. In both instances, even the biggest Rogue One enthusiasts would argue they amounted to little more than cheap fan service. Time will tell what surprises Solo has in store, but there are a few legacy characters we'd rather not see again.
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Vader's presence in Rogue One made a lot of sense - and not just for the obvious business reasons. With Gareth Edwards' film dealing directly with the Rebellion/Empire conflict, it would have been odd if the Imperial enforcer wasn't part of the proceedings. Vader almost had to be in Rogue One, and the filmmakers ensured his brief role paid off memorably. In the case of Solo, things are different. Yes, the spinoff is set in the heyday of the Galactic Empire, but the Star Wars universe is an expansive place, so Vader's absence could be easily explainable. From a story perspective, there's simply no need for his inclusion, especially since Solo is more about an exploration of the seedy underbelly.
Howard also has an abundance of classic Star Wars iconography to take advantage of with Han, Chewie, Lando, and the Falcon all featuring heavily in the marketing campaign. There's already enough nostalgic elements to complement the new aspects, so forcing Vader in too would be a disservice. There's no need to shoehorn him into Solo, and it would probably just illicit eye rolls if he strolled onto the screen. At this point in the timeline, Vader was more of a mythical figure who stayed in the shadows, so it'd be wise to honor that.
Anyone from Rogue One
Keeping with the Rogue One theme, the cast of new faces we met two years ago should also be avoided. In many cases, it'd be far too contrived. During the events of Solo, Jyn Erso is on her own committing random crimes, Cassian Andor is with the Alliance, and Chirrut Îmwe is panhandling on the streets of Jedha to spread word about the Force. Solo should be staying far away from the aforementioned religious moon and Yavin IV, seeing that its primary locations include Corellia and Kessel. Perhaps Director Krennic pops up when young Han signs up for the Empire, but even that'd be too wink-wink.
It's true certain members of the Rogue One cast actually have multi-picture deals with Lucasfilm (despite the finale where everyone died), but Solo is not the place to pick those options up. The whole idea behind the anthology initiative was to show how large the Star Wars galaxy is, and seeing one too many familiar individuals runs the risk of making it feel small. The spotlight should be on Han, Chewie, and the new Solo characters. If Jyn or another Rogue One member showed up, it would be quite distracting.
The Jedi Order was largely extinct post-Revenge of the Sith, but Star Wars Rebels showed there were still a few out there fighting the good fight. Many fans would love to see one (or more) of these characters show up in a live-action film, but Solo is the rare Star Wars movie that should actually minimize the role of the Force. Based on franchise canon, the property's most defining aspect probably shouldn't even be mentioned.
As Han told Luke in A New Hope, he's never seen anything to make him believe there's an all-powerful Force during his travels from one side of the galaxy to the other. It's possible there's something that Han brushes off as "simple tricks and nonsense," but it's definitely not necessary for this film. Fortunately, the Lucasfilm story group is in place to prevent any inconsistencies from happening, and they've shown time and time again they have an excellent handle on Star Wars. Wedge Antilles didn't cameo in Rogue One due to New Hope dialogue, so the creative team most likely won't let something like this slip through the cracks.
Han and Leia is one of the most famous love stories in all of pop culture, but their place is the original trilogy (and Force Awakens). The Princess brought the house down with a last-scene cameo in Rogue One, but much like her Sith Lord father, that made sense for the story. Rogue One led directly into A New Hope with Leia receiving the Death Star plans. It's hard to see how she could organically fit into the Solo narrative, which picks up roughly 11 years before Episode IV and ends 5 years prior to the original film.
Judging by Solo's place in the timeline, Leia is an 8-year old girl living on Alderaan when the spinoff begins and a young teenager by the time the credits roll. She didn't even become involved in the Rebel Alliance until she turned 16, so there's absolutely no reason for her to be spending time with scoundrels and smugglers in her youth. And again, Solo takes viewers through the criminal underworld, far removed from Alderaan. This one wasn't very feasible to begin with, but Han's future wife shouldn't even show up in hologram form on a HoloNet broadcast. We spent plenty of time with them in the saga. Solo is for Han's other relationships.