It looks like the first volume in this is coming out in May 2020.

https://www.penguin.co.uk/books/111/...529124583.html

The first book in a new trilogy set before Thrawn traveled to the Empire and became a Grand Admiral. Journey to the Unknown Regions and learn more about Thrawn’s origins and his home: The Chiss Ascendency.
Naturally, Timothy Zahn is writing it