Foremost, Ahsoka's namedrop of the character in her episode of The Mandalorian.

Next, Timothy Zahn, the writer of all of the Thrawn books, has been kept out of the loop: up until now.

"Zahn has said that he's not allowed to touch Thrawn's story at any point after Rebels until Filoni and his team pick it up in either animation or live-action, now."

Source: thedirect article

This points to his story being completely open for the series, which most likely was planned to be that way until the series could occur.

Furthermore, Favreau in his GMA interview confirmed that Dave Filoni was writing Ahsoka as of right now. This likely now includes Zahn's involvement in the project as they no longer are hiding spoilers relating to season 2 of The Mandalorian.

Next, the leaking community is all over a claim about a 'live-action Thrawn casting' within Lucasfilm.

I'm a huge fan of Thrawn's character, from his original Legends books, to his new canon books, and even to Star Wars: Rebels, the character fascinates me and I'm really looking forward to (hopefully) seeing his involvement in this series.

Feel free to leave a comment and let me know your excitement and speculation for the character!