Star Wars Rebels producer, Dave Filoni hints a the hidden villain in the concluding season of the animated series. He said Thrawn will be the main villain in season 4, but there may be another villain hiding in the far galaxy, which the rebels must face:
“He’s still the one that is watching for them and that they’re running from and their main competitor,” he tells the Daily News. “Which is different, we’ve never done that over two seasons before.
There may some other villains more arch than him, hiding somewhere … we’ll have to wait and see.”
Source: Daily News