[UPDATE: Felicity Jones is playing the female lead in the 2016 Star Wars spinoff.]
Director J.J. Abrams' Star Wars: Episode VII - The Force Awakens will be in post-production mode for this year, leading up to its theatrical debut in December. As such, the Star Wars rumor mill has shifted focus to director Gareth Edwards' spinoff movie, scheduled for release in 2016 (one year after Force Awakens hits the scene).
Fan-favorite Tatiana Maslany - known for playing multiple clones on the sci-fi TV show Orphan Black - is rumored to have landed the lead in Edwards' spinoff, though her would-be role in the film has remained a mystery. We're now hearing that Maslany could be playing none other that the live-action version of Sabine Wren, a character who can currently be seen on the small screen in the Disney XD animated series Star Wars Rebels.
Production on the 2016 Star Wars spinoff will get started in a few months at Pinewood Studios (where Force Awakens was shot), while rumor has it the project - based on a script currently being rewritten by Chris Weitz (The Golden Compass, Cinderella) - revolves around a team of bounty hunters recruited to steal the plans for the original Death Star.
Separate sources for Total Geeks and The Mary Sue claim that Maslany has indeed landed a lead role in the project, while IKwiz (hat tip Latino Review) has supposedly been informed by a source that she's playing Sabine, who's "a little older and [now] a bounty hunter" in Edwards' film. It's worth noting that IKwiz is also the source for a rumor that Michael Fassbender is being eyed to play Boba Fett in the movie, which has been making the rounds of late. The Fassbender story is very questionable at best, but the site's story about Maslany - who recently opened up her work schedule around when Edwards' spninoff is due to begin shooting - makes more sense.
The rumor that Maslany is playing Sabine in Edwards' Star Wars spinoff doesn't seem that unreasonable, all things considered. Walt Disney Pictures and Lucasfilm have been quite open about their planned synergistic approach with the Star Wars franchise heron out - an inter-connected storytelling approach resembling that of Marvel Studios and its "Shared Universe" - where every installment is part of the canon, regardless of the platform (movies, TV shows, comics, etc.).
Characters on Marvel TV shows Agents of S.H.I.E.L.D. and Agent Carter have also appeared in the studio's films, so it doesn't require a huge stretch to assume we could likewise see certain Rebels characters appear in the live-action Star Wars movies too. It would make sense, from a business perspective, for Disney/Lucasfilm to try and hook a young generation of Star Wars fans on new characters such as Sabine, thus expanding their brand beyond already established Star Wars players. We've heard that marketing for Force Awakens will continue to highlight its young stars for that reason, while having a Rebels character appear in Edwards' spinoff would likewise offer lots of cross-promotional opportunities.
Sabine, out of all the main players on Rebels, would make the most sense to appear in a film spinoff about mercenaries for hire (including one who wears Mandalorian armor like her). It doesn't take a huge leap to imagine that an older version of the character could be a bounty hunter sympathetic to the rebellion against the Galactic Empire - and thus, inclined to participate in a dangerous mission to steal the plans for its super-weapon. Alternatively, the (only somewhat) older Sabine could be a representative for the Rebel Alliance, who recruits said bounty hunters in Edwards' film.
Meanwhile, those who've been tuning into Rebels would get to enjoy an enriched storytelling experience, as they would already have a whole backstory of knowledge about Sabine by the time Maslany would be playing the character on the big screen. So, if this rumor pans out - and remember that's not even remotely a given right now - it doesn't sound like a bad idea at all.
Star Wars: Episode VII - The Force Awakens opens in theaters on December 18th, 2015, followed by the first spinoff on December 16th, 2016.