In an interview with Collider, Simon Kinberg had a thing or two to say about the appearance of at least one as-yet-unidentified character in Rebels:
Writing dialogue for any of the legacy characters is as big a thrill as anything I’ve ever had in my life. I said this to [producer Kathleen Kennedy] that when I open up a Final Draft document, and I tab over to “Character” and it’s a character from the original films—to be nameless until people see them—but if it’s a character from the original films, just typing in those letters and then that being the recorded name in that name database for that script is as surreal, and perhaps more so, than anything I’ve experienced in my career.
Is that character Obi-Wan Kenobi? Possibly, though that character is now almost synonymous with the prequels. It seems that Lando Calrissian is much more likely, especially given that Billy Dee Williams himself has hinted at doing Lando’s voice recently.
For what it’s worth, Kinberg is one of the key people shaping this new era of Star Wars, having his hand in Rebels, Episode VII and at least one of the upcoming spinoff films. For Rebels he’ll be writing the initial two back-to-back episodes as well as the season finale. Say what you will about his other work, but Kinberg seems to be a passionate fan who both understands the essence of Star Wars and the mechanics of mainstream scriptwriting. Working alongside both Dave Filoni and Lawrence Kasdan, Kinberg should produce some great Star Wars content.