Thrawn is ONLY at Lothal because Governor Pryce specifically ASKED for him to BE there. That is the way LFL wanted to set up the plot for Seasons 3 and 4.
Then LFL realized what Get Thrawn In campaign knew all along: Thrawn is a much better villain than say, Kallus who is a part of ISB. LFL writing group was not prepared to bring in Screed or Rancit to Star Wars Rebels or even Moff Trachta. They were not talking about the villainy of Kallus. Kallus can’t be evil. His voice, his look, his “foiling” cannot be made for fans to dislike Kallus. Bringing in Vader of the Emperor would have been TCW Redux. LFL HAD to branch out and show fans why the Empire is fucking evil.
Rogue One did a pretty good job of it with Krennic, but what happened in the end? He died, right?
There was very little antagonist like Motti and Tagge or useless bickering. And Star Wars Rebels weren’t prepared to bring in Motti or Tagge or Veers or Ozzel or dayum just bring in Lorth Needa, at least he’s in the Revenge of the Sith novelization, right?
So that was one reason why we promoted Get Thrawn In. He’s unique enough to make that villain distinction for fans to easily remember. Except some fans want Happy Rainbow Ponyland tales and well, glitter on Death trooper armor is a bit much…