Having thought about it, I'm not really sure Sylvanas would be super-satisfying as a raid encounter on a mechanical level - beyond my personal desire to see her finally die. I suppose you could draw inspiration from the fight with her Echo in the End Time dungeon, where she uses some impressive abilities with interesting mechanics. Beyond that, though; you'd have to do some serious alterations to what kind of a fighter she is to make an impressive end-of-expansion boss out of her.
That being said I'm pretty confident we will be fighting her and probably ending her decisively this time around. She may be the penultimate boss of Shadowlands final raid, guarding the Jailer with her unlife or some such.