Excited for this, but Thrawn will be ruined. The Rebels have to beat him because theyre the protagonists, but going by the books they really dont have the skill.
Darth Vader was the overarching villain of Season 2, and he was never beaten by the protagonists of Rebels.
I'm not sure where the worries about him being ruined come from. Maul, Vader, Tarkin, Ahsoka, Rex, etc. weren't ruined by the show. Most of the disappointment with Rebels comes from the original characters.
He could be the over arching villain who when he shows up wins every battle until the one in the last episode where the Rebels barely get a victory which causes the Emperor or the various Imperial governors or officers to use it as an excuse as to exile him to the unknown regions due to him being the rare alien in a mostly human run Empire. Doing that would allow them to use him later down the line, even potentially showing up in a sequel trilogy film or spin off series as a major villain.