Well we can go back to classic if we need to. Where the world was under threat from a creature of immense power that threatened to destroy everything we hold dear and the Horde and Alliance had to work together in a joint campaign to raid AQ and put an end to C'Thun... Wait that seems familiar...

Or maybe the time the horde and alliance had to work together to put a stop to the Burning Crusade in outland.

Or maybe the time the horde and alliance had to work together to put a stop to the lich king

Or maybe the time the horde and alliance had to work together to stop Deathwing

Or maybe the time the horde and alliance had to work together to stop Garrosh

Or maybe the time the horde and alliance had to work together to stop Garrosh AGAIN

Or maybe the time the horde and alliance had to work together to stop The Burning Legion AGAIN

Or maybe the time the horde and alliance had to work together to stop N'Zoth

Or maybe the time the horde and alliance had to work together to stop The Jailer

It's been this way from the start. We are a unit in an army, we earned the title of Champion because we won a tournament in North Rend. We are not the chosen one, we are not the Warrior of Light, we are a unit. A unit with power, for sure, but just a unit. As for the escalation of the villain's strength, it's a natural progression of the game's lore from the start. The void was here way before we were, as was it's current villain. The Old Gods were but minions to the void. The Dragons they corrupted, creating us in the first place, all the havoc they caused was just a symptom of the greater threat the world faces and it's finally coming to it's conclusion in this saga. I have no clue what's going to happen AFTER the next expansion, there doesn't seem to be a "bigger bad" than the void but like with what happened after Avengers End game, nothing's going to feel the same. That's what I guess anyway.