Sylvie is on her way to a form of redemption and a definite chnage in character - now whether that turns out to be a great thing orn ot, depends on how well it's written, but her days as the love/hate character, the 2bitch" female character most shows and stories have seem to be over.
Who would replace her?
Elune? - I doubt it, she is benevolent despite a "?" mark over teldrassil
Zovvaal? I don't think you can really put god-like beings into that sort of role - He's more Sargeras level type - wait, isn't Zovaal more powerful than Sargeras? Presumably Eternal Ones > titans
My bet, is Azshara -those two sly females had quite the catty moments in N'zoths revelation of their conspiring with each other. If Sylvie is turning good, then Azshara is the ideal candidate to take up that mantle.. I can't see anyone else filling that role - Tyrande possibly, until it is clear she chose renewal over vengeance and so gets to keep her night warrior powers but as a high priestess good person.
They could bring Xal'atath in play again, there is something more devious about that character, but not everything seems as evil as it appears, same with Azshara and same with Sylvie, so she might work.
Who is your bet?