At Star Wars Celebration Europe, fans got a huge surprise during the Rebels panel, when executive producer Dave Filoni revealed an extended trailer for Season 3 that revealed a new villain: Grand Admiral Thrawn!
Thrawn, of course, is a fan favorite from Timothy Zahn's acclaimed "Heir to the Empire" book series. With the books no longer canon, fans had hoped Thrawn would become "official" again in some way. Filoni also revealed that we will be getting a new Thrawn book in 2017, written by Zahn:
The new season of Rebels just keeps getting better!