When people (like me) complain about the loss of the EU and the refusal of Disney to allow new stories to be told in the "Legends" continuity (why? would love a real explanation), it is because of novels like the Thrawn trilogy. I remained a fan of Star Wars because of the EU and the 1990s to 2014ish novels. Not because of the movies. So while seeing Thrawn being re-introduced into canon is great, its also a constant reminder of Disney's bizarre policy of blocking off the EU and allowing no one to play in that sandbox again.
Below is the trailer and Bleeding Cool has the summary of the Rebels Panel.