The Broken Shore scenerio made me think a lot about Varian, past history, and Blizzard's in general story telling.
Given this, I believe that Varian will be the Avatar of Sargeras and the final boss of Legion. Wild accusation? Absolutely. But here is my reasoning:
++Varian is not shown to be killed, an explosion occurs around him. Since Varian is not directly shown to be killed, this absolutely allows him to make a come back.+This is not the first time a hero is thought to be dead. Bolvar was thought dead, but eventually became immortal and the new Lich King. This also sets up the plot-possibility that a creation destroys its master (as the Legion, propgated by Sargeras, created the Lich King).+Gul'dan "embeds" Varian with an orb. This can be percieved as a bomb, but this can also be percieved as an impregnation of some sort. This would not be the first time that someone has been infused with the spirit of Sargeras, as Aegwyn was impregnated with the power of Sargeras which carried into her son Medivh.+If Varian becomes the Avatar of Sargeras, he would be a raid boss, specifically the end all boss of Legion. This would cause players to once again fight a race leader, and make the score tally 1 to 1 (Garrosh for horde, Varian for alliance). Jaina being a Dreadlord would throw this for a loop. Unless you consider Jaina and Nazgrim on the same tier.+There was a hidden scenerio that involved Stormwind Harbor, and Stormwind City, burning from fel flames. This could be a foreshadowing of Fel-Varian, Avatar of Sargeras, returning to his home and destroying the remaining resistance of Azeroth. Effectively an inverse Siege of Orgrimmar.What more would force Anduin to grow up, then slay his own father, the former King. This also allows the possiblity of the Avatar of Sargeras "having Kings blood" be stronger, and would also follow the idea that Sargeras wants to posses a strong corpse (Medivh was the Guardian!).