They'll recast Baylan. They have to because his arc is far too important. In some ways, it seems more important than the "main" Thrawn plot. No doubt, Ahsoka and Sabine will be dealing with him in season 2 since they are trapped on the same planet with him.

Now we know Baylan was indeed hunting the Father/Son/Daughter, but I sure would have liked to have gotten more. Leave 'em hungry, I guess.
There has to be significance that he was standing on the arm of the Father rather than on either side. It seems he does want to transcend Dark and Light and derive power from the "source" of both. What his plans are is anyone's guess at this point, but he'd certainly become far more powerful than Palpatine ever was if he can tap into that power. Really looking forward to see where Filoni goes with this.

If nothing else in this series screams out that one HAS to watch Clone Wars and Rebels to understand what's happening in Ahsoka, it's this finale episode. So much happened here that absolutely requires the knowledge of seeing those two series, I can't imagine anyone who hasn't watched them being utterly lost watching this episode.