Thinking about it, her becoming the Arbiter/the Jailer would NOT BE BAD.
Because, by far, in the grand scheme, she was mostly serving herself even since the Jailer saved her from that Icecrown jump and took hold of her using her fear of being unfairly doomed to an eternal damnation in the Maw for something that (unknown to her) HE HIMSELF had essentially caused. Pretty sure that Sylvanas that pulled everything that she did in BFA would NOT agree to that as it would essentially put her in prison, where she has to do what she MUST instead of what she WANTS (it was shown numerously that it's essentially her trigger button). If re-gaining the WHOLE soul with Uther meant anything, it would make her realize how selfish her actions were in the end and decides to do something she wouldn't after her deal with Jailer - sacrifice HER freedom for the sake of others.
Correct me if I'm wrong here somewhere.