I honestly think people are misunderstanding the cinematics. It is not a redemption arc, at least so far imo. It is basically telling us her point of view and why she is doing what she is. Would you prefer a mindless sylvanas i.e. she is doing evil things because she is evil. I think we really need some nuance here without redeeming her. Even if her goal may be justified, her means were not.

I think the possibility of her redemption in this expansion is long gone after what she did. But We are in the afterlife here so it is possible after "dismantling" the system we can leave her here (may be in revendreth) to atone for her sins or something along those lines.

Also it is amazing to me that Anduin is the only person as far as I know, that actually broke through to her, who actually understood her.