Will be Sylvanas. Calling it now.
Sylvanas will sacrifice her body to save azeroth during the zovaal encounter to use it as a host for the world soul.
Sylvanas in turn will either face oblivion or be sent to the shadow lands to await judgement by the new arbitor. Btw Incase some aren't aware the new arbitor has been datamined. I'll leave it to you to search who to avoid spoiling anyone.
But yeah, that fake titan Sylvanas pic that's been floating around will end up being real in some way.
So yeah, in the end azeroth will finally manifest physically with the help of Sylvanas body.
This way we get to keep Sylvanas and keep the fan Bois happy, but yet not really to keep the nelf fan Bois happy.