Today it was announced that Lucasfilm is developing a new live-action Star Wars TV series for the Disney+ streaming service - one that will serve as a prequel to Rogue One: A Star Wars Story and follow Cassian Andor (Diego Luna), the Rebel Alliance captain / spy.
Rogue One remains one of the more controversial entries in the new era of Disney/Lucasfilm Star Wars movies, and no doubt there will be a lot of fans who are skeptical about (if not hostile toward) the idea of Cassian Andor getting his own series - especially one that's essentially a prequel to a prequel. However, there's a lot of potential for Star Wars to build something great with this Cassian Andor TV series, based on what we already got from the character in Rogue One.
Rogue One's big sell was its climatic final act, which filled in a crucial piece of Rebel Alliance history. That history in question was the mission to steal the Death Star plans from a secure Imperial stronghold on Scarif, and in doing so give the Rebel Alliance the small glimmer of hope that would eventually lead to crucial victories of Rebellion heroes like Luke, Leia and Han Solo. It was definitely a grand spectacle, and conveyed a powerful theme about the hand of fate, and how thin the lines between success an failure or bravery and despair can be, in dark, dire times.
Of course, Rogue One also gets criticized for being something of a disjointed film, with the third act famously having to be revised and re-shot to get the theatrical version we saw. The first two acts of director Gareth Edwards' film was something much darker: a story of how the war between the Rebellion and Empire was consuming those fighting it, on both sides of the conflict. Cassian Andor served as the major poster child for that thematic arc, starting from when we meet him in Rogue One, as he solicits information about the Death Star from a spy contact at the Ring of Kafrene trading outpost, and then cruelly murdered the contact, to tie off that loose end. That thematic line continues throughout the film, as Cassian must grow beyond being a "by any means necessary" war dog who takes whatever order is given, and become a leader and hero willing to hold himself to a higher moral standard, and lay his life on the line for the cause of hope, rather than cold, calculating strategy.
Throughout Rogue One we are given sketches of Cassian's dark life story - how he lost his family at a young age like Jyn Erso, and how to grow up in hard world of lies, espionage, and killing, which made him the morally compromised monster he sees in himself when finally staring down his sniper's scope at Galen Erso. In short, Cassian is the most effective character at conveying a theme Edwards was exploring in Rogue One - and one that will be a great foundation for this Rogue One prequel series.
That thematic idea is an exploration of how the struggle between the light and dark sides of morality (if not The Force) extends to non-Force user characters in the Rebellion and Imperial military campaigns - essentially a "war is hell" theme of moral corruption and brutal sacrifice, which had never really been a focus of Star Wars' fantastical lore beforehand, but has since become a big part of it (see: The Last Jedi). The world that this Cassian Andor series will expand upon will be fertile ground for that theme to be further explored, as we witness the lonely boy that is Cassian slowly evolve into a hardened, war-torn spy, one dark compromise at a time. Thanks to shows like Breaking Bad, those kinds of slow descent into darkness character stories have become all the rage - something the powers that be behind Star Wars likely recognize.
If that doesn't do it for you, the obvious avenues of tie-in and crossover with other popular characters of the OT prequel era (Han Solo, Darth Maul, Vader, Jabba, Saul Gerrera etc.) is a novelty that will also keep fans invested.