Star Wars Rebels are going to be up against my very favorite Imperial Admiral, Grand Admiral Thrawn. He's humanoid, but not human, as he has blue skin and red eyes. He is also a brilliant tactician. I wrote a review of Timothy Zahn's 1990s trilogy of Thrawn books. You can read it here.
Thrawn has become something of a cult figure in Star Wars fandom, with plenty of fan fic stories of varying worth. One I have enjoyed is a trilogy, originally written in serial form, called Daughter of the Empire. It's a true science fiction romance, with Thrawn as the male MC. Be warned, the editing is... poor, to put it nicely. But if you can train your brain to ignore the errors, the story is a lot of fun for die-hard Star Wars fans. I have read it three times. You can take a look here.
Anyway, back to Star Wars Rebels. It seems GA Thrawn will make his appearance when Rebels series three premiers in late September.
And this is the really sad bit. One reason I signed up for Netflix in Oz was Star Wars Rebels. Except the 30% of the content in the US which is offered in Australia does not include SW Rebels. Which is one reason we no longer get Netflix.