Since when is "badass soldier" a "black stereotype" ?
Also Whitaker's character is Saw Garrera from clone wars :
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You dont really see it in Rebels either.
The Imperials are still saturday cartoon mooks.
Of course that is a given, it is a cartoon after all.
Clone Wars showed a bit more grit but that was because there were many jedi characters and there were a lot of good guy mooks to die and show the strength of the enemy.
Since Rebels follows the adventures of a team exclusively comprised of protagonists that is unlikely to happen.