We all know the end of WoW is coming, and will end with something akin to fighting Sargeras in the Twisting Nether and destroying the Burning Legion once and for all, but the circumstances leading up to that are still pretty mysterious.
Some breakthroughs in Lore have occurred lately especially finding out that Illidan is the Naaru's chosen warrior of Light and Shadow etc. With this in mind I have created a few theories as to how the rest of Azeroth will come together to support Illidan as he leads us to the end.
While it's clear Illidan is the "chosen one" it would be very anti-wow spirited to have him win the day by himself, and there will likely be many supporting characters joining Green Moses in the journey to the Nether, one of which we can all assume is Anduin. Now at the moment he's not a particularly impressive fighter, but he is a priest which leads me to one theory that seems pretty obvious for me.
Theory 1: Anduin will become a Paladin and wield his Father's sword, with which he will unite both Alliance and Horde.
So the transition from Priest to Paladin seems pretty cut and dry, especially considering Anduin's father. There have been instances in comics where Anduin and Varyian have been training together with swords so he may already primed to take a frontline role in the Legion conflict.
It's also possible that Anduin receives some sort of blessing from Wrathion, the Black Prince, as he saw enough potential in Anduin to accompany him in MoP and can most likely see that he is capable of bringing Azeroth together, something his father would likely not have as Varyian had a distaste for Orcs for a long time.
He was also trained by Prophet Velen, which may indicate that Velen knew Anduin was destined for greatness and may have even been tasked by the Naaru to train him. It is possible that a very "Qui-gon X Anakin" situation occurred that set that in motion.
Theory 2: The Sons of Lothar will return in full.
This theory is a bit shaky as the fate of Turalyon and Alleria is undetermined, even the Marksman hunter quest to retrieve Alleria's bow hints that she is missing in the Nether, and we know that the 2 are connected heavily. There was also a rumor that Turalyon may have died fighting the Legion and sent a letter to Arathor, his son in Outland, detailing his fate.
However if there's 1 thing we know from fighting the Lich King it's that the power of the Light is pretty immense, especially in desperate times. It wouldn't surprise me if both Turalyon and Alleria lived through their Demon hunting adventures and return via help from the OG son of Lothar, Khadgar, especially considering Khadgar's current powerlevel. All that would remain is for Kurdran Wildhammer and Danath Trollbane to return to the fray and the legion hunting supergroup would be back to full strength.
And lastly the true tinfoil hat theory. Theory 3: Thrall will return to glory and summon the Elemental Lords themselves to fight the Legion.
Thrall is currently in a depressed state, as not even Doomhammer will respond to his call. He's going to probably spend most of Legion taking a backseat and brood while we with our Artifacts fight back the Legion invasion. This will give him time to cope with his mistakes and most likely through the help of Aggra and his son Durak overcome his depressed state. We already know Thrall has the potential to power up to the level of the Dragon Aspects, its time for him to go 1 step further and play with the actual Elemental lords, which would be nice since after the "deaths" of Al'akir and Ragnaros we don't actually know if there are new Lords of Fire and Wind.
This theory is just what I would LIKE to happen, I would love Green Jesus and Green Moses to tag-team Sargeras in some epic final battle, but we'll remain in the dark for a while.
I know it's a long read but thank you anyways.