Just a thought I like to share and discuss by writing as if it had happen already.
One way or another the Burning Legion will kill the King of Stormwind, Varian Wrynn.
Anduin now swear to avenge his father's death as he takes the crown and becomes the new King of Stormwind, but who will be the High King\Queen of the Alliance?
This is now the time for the Alliance to shine in the beginning of an expansion.
Anduin has shown to be a man of high intelligence but also very thoughtful, peaceful human being.
But as he will revenge his father's death and stopping the Burning Legion from destroying Azeroth completely.
You will see a new less peaceful side of Anduin, the King of Stormwind.
Anduin Wrynn, the only student of Velen, the Great Prophet. Will fight side by side with his mentor Velen, against the Burning Legion as Velen's former bestfriend, Kil'jaeden is leading the attack personally against Azeroth. It's quite personal. All of it.
Imagine hell and heaven, you have the "angels" (Anduin & Velen) walking thru the flames of hell in Kil'jaedens own "home\domain\whatever place he is", with determination they will destroy him! So not in Azeroth, not in some other planet the Burning Legion wants to take, but in Kil'jadens own backyard, they march up to destroy him for good.
That's at least my vision for the future, what do you think?