The end boss is going to be The Daily Quest. It will claim a great many lives.
And Blizzard never, ever, ever ever ever ever said Illidan was the final boss in TBC. He was in the cinematic because he's a well known and established character and he was a major part of the expansion, but everything in it was leading to us fighting Kil'jaeden. People claim that SWP was tacked on at the end because they read that somewhere, but it's just not true. You don't hurry up and rush and make an emergency raid, especially one as well made as SWP that had an entire staggered daily hub surrounding it. Kael'thas even made mention of his people summoning his true master before Black Temple was even released.
Even the name of the expansion. The Burning Crusade. We were fighting... the Burning Legion.
Plot twist: Garrosh and Jaina fuse together and mighty morph into a hideous revenge monster of doom. There's your final boss.
Idk if this would work as a mechanic but it would be pretty cool if the last boss was Jaina vs Garrosh and we have to try and stop them both destroying everything and killing each other.
I guess it would fail because its ultimately gonna be an add fight which would make a pretty lacklustre final encounter.
Unless it starts with us having to control the adds (Jaina and Garrosh's forces) then maybe every so often one gets the other down and we have to fight off a killing blow, then they continue fighting we do some more adds until the final when Garrosh almost kills Jaina but someone like Thrall or Varian stops and kills him.