Disney successfully re-launched the Star Wars brand in 2015 with Star Wars: Episode VII - The Force Awakens, but they have much bigger plans than just exploring stories in the saga films. Comics, novels, and shows like Star Wars Rebels each add to the official canon on a regular basis, but in order to further expand the galaxy far, far away, Disney and Luasfilm are experimenting with anthology films. Rogue One: A Star Wars Story was the first test to gauge interest from audiences in a story that largely features unknown elements.
One of the new characters fans got to know in Rogue One was Diego Luna as Cassian Andor, a morally-grey member of the Rebellion. It is made fairly clear that Cassian and the rest of the Rogue One crew do not survive the film, leaving only events before the movie open as possibilities for their return. This is the believed timeline for the young Han Solo movie, and Luna's reaction after being asked if he'll appear may indicate something is in the works.
MTV (via /Film) had the chance to interview Luna at the Golden Globes and asked him about the potential for Cassian to appear in anything moving forward (in reference to the slate and not necessarily the timeline). Instead of flat out denying that it could happen, Luna tries to play it off as a ridiculous notion he knows nothing about, but his poker face may say otherwise.
Based on the timeline, Han Solo would be one of the only projects that Cassian could appear in and fit within the canon. However, Cassian appears to have been involved with the Rebellion for most if not all of his life, so figuring out a natural reason for him to cross paths with Han Solo and company could be difficult.
Luna's portrayal of the character was liked by most, with Cassian being one of the characters fans want to know more about. Han Solo could be the avenue they take to make that dream a reality, but they'd have to figure out a way to incorporate him in organically so it does not just feel like fan service. In any case, this is hardly confirmation that he will appear in the film or that they are discussing it, but Luna's reaction will be the big giveaway should this turn out to be true.
Outside of Han Solo, the avenue that would make the most sense for Cassian to cameo in would be Rebels at some point. The show just had Forest Whitaker play fellow Rogue One character Saw Gerrera, so the series very well could bring Cassian in as well. This would be a much more natural fit on paper anyways as Cassian and the Ghost crew are both vital members of the Rebellion. Time will tell whether or not Luna reprises his role in some form, but based on this interview alone, he does not do much to try and dismiss this idea.
