Zahn recently teased a new character from Treason:
"His name is Assistant Director Ronan. He is one of the assistant directors on Stardust, extremely loyal to Director Krennic, not really liking Thrawn much at all [...] His cape is thigh-length, not full length. He's only an assistant director."
When asked if Captain Pellaeon would make an appearance:
"It remains to be seen. Now that he's canon, he's fair game."
He also commented on the vastly different tactical approaches taken by Thrawn and Krennic respectively:
"Krennic is all about power, while Thrawn takes a different focus to getting things done. Thrawn would prefer a surgical strike with the minimal amount of necessary force to Krennic's all-out destruction."
Zahn went on to say that he has plans on pitching more Thrawn-based novels to Lucasfilm/Del Rey:
"I would like to do more. That's going to be up to Del Rey and Lucasfilm. But the problem right now with Treason, we will have two separate Thrawn trilogies. This is going to be very confusing. We need to add a fourth and have a Thrawn tetralogy. I will definitely be pitching a fourth book, maybe a fifth, and maybe a sixth depending on how things go. As long as Del Rey is willing to buy them and you guys are willing buy them, I've got a lot of stories... It's always at the pleasure of Lucasfilm ultimately, and Del Rey, Penguin Random House, whether they want what they want from me if anything. I'll say it again. If I'm never asked another Star Wars book, I've had my say. I've a lot of fun and I'm content."
And given the success these novels have had so far, I imagine they'll allow him to do pretty much whatever he wants... So hopefully we'll find out what became of Thrawn after the events of Rebels in these forthcoming projects. It's a story that I only trust Zahn to tell, tbh.