The Official Star Wars Youtube page has shared a Star Wars Rebels video! Executive producer Greg Weisman discusses working on Rebels, his favorite Star Wars movie, and how he made it in the industry:
Screencaps:
Alderaan? Lothal! (Thanks to Star Wars Always for reminding me about sienarfleetsystems.com/)
AT-ST:
Lothal architecture?
Star Destroyers, The Wolf, and more:
Interior of the Ghost?
Speeder concepts:
Hanger concepts? Perhaps the rebels have a secret hanger? 
Is that a Sandtrooper? Stormtrooper gun concepts as well as another gun concept pointed out before:
A good example of the index card story outlining method.
Stormtrooper concept art:
Character concepts. One of them appears to be a little Astro droid.
Pablo Hildalgo next to a clearer image of the Ghost concept art:
Story boards:
Filoni drawing during a meeting:
