Metzen recently said that he wants to see Anduin become King and that every now and then you'll see his model change to reflect his growth in age. Anduin can't become King if Varian is still breathing so how long before he starts pushing daisies? I doubt it happens in WoD, the expansion after? the one after that?
Does the Horde kill him? Sylvanas rips him and raises him as a forsaken? Maybe Wrathion thinks he's not the guy to lead the Allinace against the Legion?
My guess would be that Jaina goes full Batshit Human Supremacist Nazi and kills Varian in a power grab.
I highly doubt Jaina is going batshit - they made it clear in the book that she was tired of being peaceful, but not out to wipe out an entire race. They've had enough main characters go that route, I think it's been played too many times.
Idgaf how Varian dies, I never liked his story or how he was introduced. The comics we're great but still felt very, I don't know, "me'dan".
// Thanks to maybenotquiteasheavy for the Signature & Avatar! //