Both Making Star Wars and TheForce.Net have posted some interesting details regarding Rebels. It seems that various people are getting to see the double-episode debut written by Simon Kinberg, and so details are starting to leak.
Firstly, and most intriguingly, Lothal is apparently the home of a Devaronian crime lord called (Chatro) Vizago. This already adds an interesting element beyond the insurgents vs. the Empire angle we’ve seen thus far, and also ties into Star Wars: A New Hope, given that that was the only time we saw the Devaronian species in any of the films.
Secondly, the spice mines of Kessel will feature somehow. This is another nice nod to the original Star Wars.
Thirdly, the Inquisitor (voiced by Jason Isaacs) is not seen more than we saw him in the trailer (which was as a hologram). Agent Kallus (voiced by David Oyelowo) will be the main villain. This is a nice touch—when we see the Inquisitor in the flesh, it’ll have that same build-up of anticipation behind it that accompanied the Emperor’s appearance in Return of the Jedi.
If I had to guess, I’d say that the Inquisitor only appears a few times as a physical threat in season 1 of Rebels, and the show will progress up a hierarchy of villains. The first season or two will centre around Kallus reporting to the Inquisitor, while later seasons will have the Inquisitor reporting to Darth Vader. Finally, as the Death Star plans are stolen in the final season, Vader will be the main villain.