We don't know anything about story and final villain of Warlords of Draenor, and I'm hoping they don't cave and tell us.
I like not knowing the end of the story before the book is even released.
Wrath was a good set up, but I felt like Arthas appeared SO MUCH he lost a lot of the menacing aspect he's supposed to have had. He became pretty cliche by the time you fought him. Just a handful of the appearances could have been kept - I liked the ones that he empowered minions from afar rather than the ones where he was physically there and just walks off to let us die in an easily escapeable trap and assume everything went according to plan.