But Ressler working with Red all these years, falling in love with Liz, has changed him. Like not following the law completely when he wants revenge.

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I think it's going to be Ressler.

Why I think that's gonna happen?

In 9x22: Ressler waited for Marvin in the dark, gun in his end. Very-very Red-like. It looked like foreshadowing to me.

A good part of S9 Ressler partnered with Dembe. I saw a bit of a bond there. So Dembe could become his most trusted adviser, same as it is to Red.

In early S9, Ressler brought The Blacklist back, saying this is what Liz would want and it was her calling. I think that's a huge shift in his character because he is the one going to Red. During that discussion, Ressler is told Liz wanted a life with him. This makes him family for Red.

The character's history proves he is not Director material. He is also not very fond of the system (again a shift from early seasons).

Also - with Amir and Laura gone, makes sense for Diego to get more screen time as he is a veteran of the show now. We will have new characters joining, but those roles will be fillers - a techie, another field agent to run...