Spoiler for those who didn't do the new raid.
I am referring to, obviously, everyone's favorite Banshee Prince Nathanos.
In the transition phase of Jaina fight, we free Nathanos from an iceblock. After that, he chides us for not finishing off Jaina yet, then stay afk and do nothing until Jaina escapes, when he shows up and do his usual "How could you let her escape? Sylvanas will be pissed, but oh well gz"
Since there's a distinct lack of other notable NPCs in that fight, I think it's safe to say lorewise, Nathanos will be credited with defeating Jaina in BoD.
Seriously, why is he EVERYWHERE yet doing absolutely nothing in game except snarking at us? Why couldn't we get Talanji confronting her father's killer? Why couldn't we get Rokhan and Rexxar to face Jaina?
At this rate, he will be the leader of the expedition in the Azshara patch, and he will be the one who kills Sylvanas in the end, regardless how little or how much sense that would make.
P.S Jaina on Prismatic Disc looks awesome