As an aside, I don't think Sylvanas will end up being a raid boss or even killed in Shadowlands. I think she's too popular for that. What I think might happen is that she'll end up taking the Jailer's place in the Maw, becoming some sort of "master over death" entity. In this way she could never be trapped in whatever "hell" she foresaw when she leapt from Icecrown because she would become the one responsible for sticking souls in those "hells".
In this way Blizzard gets to keep using her (since she's popular) and avoid raid bossing yet another Horde faction leader. They could also then say "See? She's not Garrosh 2.0!". Obviously that's all pure speculation on my part.