Walt Disney Studios and Lucasfilm's Star Wars universe will include new live-action movies, animated TV shows, novels, and comic books that all form a cohesive multi-platform Shared Universe - meaning, there's potential for Star Wars characters/events from any medium to cross-over with others. That's given rise to speculation that we could see, say, the main players on the Star Wars Rebels animated series make the jump to the big screen in an upcoming movie.
Gareth Edwards' upcoming Star Wars Anthology: Rogue One - which, like Rebels, takes place not too long before Episode IV: A New Hope - seems an ideal candidate for a Rebels cross-over, and there's even a (almost certainly untrue) rumor that Orphan Black's Tatiana Maslany will play Rebels character Sabine. However, even if Rogue One contains no references to events and/or characters from Rebels (or vice versa) it doesn't mean that events or characters from Rebels won't ever play a role in a future live-action feature.
/Film, when it attended the 2015 Star Wars Celebration, interviewed Dave Filoni and Simon Kinberg - who co-created Rebels with Lucasfilm Director of Creative Content Strategy Carrie Beck. Kinberg confirmed there have been talks about having the Star Wars animated TV shows and live-action movies cross paths, while Filoni offered the following response when asked specifically if Rebels characters could show up in either future Anthology or Episode films.
I would never say never to that. I mean, now that Lucasfilm is so integrated in story from top to bottom in all different forms of media, we already have had our characters cross over into novels. We’re gonna have our crossover into comic books. I would say it’s entirely possible. And I would encourage it.
I’ve talked to the creators of the other films going on and, you know, I’ve always made it clear if you want somebody, just let me know. They can jump over there. I don’t care. I’m all for it. Because I think it makes the experience for fans so much stronger. And what a moment it would be to see some of these animated characters up there live on the screen. It would be great for me too.
Kinberg also noted that upcoming Star Wars stories must "acknowledge the reality of [whatever's] happening in that moment in time." It's very much like the situation with Marvel Studios movies and TV series: they take place in the same universe and (to some degree) must acknowledge what's happening elsewhere, which sometimes even results in a supporting character crossing over from big screen to small screen (or vice versa). That's also already happened on Rebels season one, which included memorable appearances by the likes of C-3PO, R2-D2, and Lando Calrissian.
However, because the Star Wars universe is so massive, it's all the more believable that key characters would never cross paths (despite them being aware of important things happening elsewhere). So, as much fun as it'd be for hardcore Star Wars fans to see Rebels characters like Sabine or Hera be portrayed in the real world in a live-action Star Wars film, it's doesn't require a huge suspension of belief to accept that it might never happen (or to accept that characters like Ezra and Kanan might still be alive during Episodes IV-VI despite never being directly mentioned).
Though, just as Marvel TV shows - be they on ABC (Agents of S.H.I.E.L.D., Agent Carter) or Netflix (Daredevil and the upcoming Defenders series) - have been more closely-interconnected with one another than their films counter-parts, the same is starting to become true for Star Wars cartoon shows. The Star Wars Rebels season two trailer revealed the episodes ahead will even include appearances by characters who were previously only part of the animated Star Wars: Clone Wars series (which Filoni also worked on).
The short of it: it doesn't sound like there are plans for Rebels characters to appear in live-action form just yet; but, to paraphrase Filoni, it's best to never say never.
Star Wars Rebels season two will air on Disney XD later this year.