The Wrynn dynasty has been through the ringer in the last few decades. First, the noble Llane Wrynn was murdered by Garona Halforcen - a person he had thought was a dear friend (though honestly, I don't really understand how the timeline works here. How long did they know each other? How long was the First War?) Varian's wife, Queen Tiffin, was killed in a riot after Stormwind was finally rebuilt (and yes, this is a bit of a fridging situation - Blizzard's got a good story, but they do sometimes fall into "Boy's Club" habits.) And then, after all that crap, Varian got kidnapped and spent years in prison, and then apparently got his personality split into two separate people and... look, I don't read the comics.
Anyway, at this point, things are actually going pretty decently for Stormwind and the Alliance. With the Horde defeated (or at least forced into a change of management) after the Siege of Orgrimmar, and the Iron Horde stopped dead in its tracks, the Alliance has had a few years to take a breather, work on rebuilding its infrastructure and consolidating its territories on Azeroth.
Varian and Anduin also seem to have reconciled their differences - Varian wanted Anduin to be a strong warrior like him, but Anduin is a more thoughtful and patient type. But Varian has managed to accept that his son is not necessarily what he had expected, and that that's ok.
We know that Anduin will be one of the major figures in Legion - he's listed on the official website. But Varian will not. Yet Varian appears in the announcement trailer, and the single screenshot we saw of the Legion cinematic showed Varian... so...
This doesn't bode well for Varian Wrynn.
Legion is going to be a big deal. After the "stakes, what stake?" problems of Warlords, it might be hard to grasp, but Legion will be the highest stakes expansion yet. This is the straight-up Burning Legion invading with a bigger force than they had in the War of the Ancients. This is their biggest play they've ever made for Azeroth, and the Legion only makes big plays (ok, to be fair, Varimathras' coup in Undercity wasn't a huge play - more of a strike of opportunity.)
The point is, this is what we call a hero-killing moment. Blizzard basically has to kill off major characters here in order to make the stakes clear. Tirion Fordring is more or less a dead man, and while Thrall might live, he's made enough bad decisions that have bitten him in the ass that I could see him going catatonic in the face of this threat - great alternative, right?
I have a strong feeling that Varian will die in the initial invasion.
But this will do a lot to invigorate Anduin Wrynn. Prophet Velen foresaw Anduin leading the Army of the Light against the Burning Legion, and it really seems like that prophecy is on its way.
I do wonder, though, what that means for Anduin in terms of his class. So far, Anduin has been a Priest (he's even the Hearthstone Priest hero,) but in the vision, Velen sees him as a Paladin. Leading an army is, after all, more the kind of thing a Paladin would do, and in a way, it would fit for him to kind of merge his own Priestly identity with the Warrior tradition of his father. Also, it just seems like the Alliance ought to have a Paladin as its leader. That said, I could understand a lot of Priests getting upset about that.
But if Anduin becomes the King of Stromwind (again - remember that during Vanilla and BC he was King, with Bolvar Fordragon as his regent,) and potentially High King of the Alliance (a title I don't really like, because High King suggests that they're all Kingdoms, when really only Stormwind and Gilneas have Kings... well, Ironforge does, but he's a big rock at the moment,) that could mean some really odd things for Alliance/Horde relations. Anduin has always been a strong advocate for peace with the Horde, and has held fast to that conviction even after the atrocities committed by Garrosh.
But perhaps we're getting ahead of ourselves. The Legion's the big matter at hand. We don't even know what Azeroth is going to look like afterwards (I'd assume it survives, given that I don't think they're going to just shut down the servers after the expansion's final boss is defeated - even if they lose another two million subscribers, it'll still be the biggest subscription-based MMO in the world, and that means more expansions will follow.)
Anduin has grown up before our eyes in-game, with the announcement trailer making it look like he's finally a real adult. He might not be up for slaughtering hundred of Orcs en masse, but I'm eager to see him blast a bunch of demons back to the Nether. Expect upheaval in Legion, and perhaps the deaths of kings.