Putting Thrawn in episode 9 would be retarded. It makes no sense and it also puts a heavy risk of destroying yet another quality character. Splitting 9 into 2 movies would also just be admitting defeat by telling us all that TLJ was so bad they had to extend the planned trilogy to attempt to save the storyline.
TLJ had absolutely 0 fan service. In my opinion, they went out of their way to give every fan of Star Wars the giant middle finger in that movie. It seems JJ understands that to some extent, but now he's going way overboard trying to "add" fan service to episode 9. "Who can we bring back. Lando? Ok, who else, Thrawn? Does that make sense? No? Whatever throw him in. Who else can we get? Can we resurrect Boba Fett? How about we add all the Jedi ghosts, let's get Qui'Gon & Obi and Yoda. What about Mace Windu, what happened to him? Let's get Samuel L. Jackson on the phone..."
They already made their bed with TLJ, and showing us exactly how they feel about the original characters. Sorry, but you're going to have to carry episode 9 with your episode 7 & 8 "heroes", and add in new characters in episode 9 to continue the story. You can't depend on beloved characters to save you when you've already tossed the majority of them away in your previous movies. Don't know how to do that? Well, figure it out, that's you're job. I don't know how you'll do it either, and quite frankly, I don't think it's possible. Prove me wrong though.