The most logical and interesting route to go for going forward, which they definitely won't do is for her to be unable to keep pushing through. Anyone who's failed so totally at their life's work and now also has to cope with ruining her own life and everyone else's for nothing should be a neurotic wreck. So put another way, her most natural character evolution is BFA Saurfang.
@Nerovar
She's redeemed because she's inhabits the same story that had Uther be lectured about how he shouldn't really be mad at Arthas and should've showed him more compassion. The framework of the story does not allow for any other conclusion but for her to get an out of jail free card. Hypothetically her having her sense of empathy reinforced after finding out she wasted the last 10 years of her life and nearly doomed everyone else could still be a follow-up on the same character we've had up to now. It won't though, for the same reason Jaina coming to terms with herself in BFA and being accepted by her kingdom was as poor as it was, namely that the story can't allow for natural human reaction because that would undermine the message. Jaina could develop and change as a character as she accepts the view that her father was right and her people reaccept her on the basis of this conclusion - it'd be 'bad' but it would logically follow from who these people are, the situation they're in and the events that happen around them. Sylvanas will be the same, except multiplied, much like her boss fight had the same problem as Jaina's, but worse.