This isn't really based on solid proof or something, but I have a feeling(hope?) that it would be about us, the nameless 'Heroes'
I've never thought about it till recent Kilrogg Episode, but when I saw that Kilrogg's death with Alliance/Horde armies behind him, I have begun to think about it
While many people(including myself) predicted that it would be Gul'dan or other Warlords, it would be such an obvious choice, which made me curious. There is a reason when Blizzard is holding their tongue tight, and it is usually a pleasant/unpleasant surprises. And Gul'dan or other Warlords, while their story can be attractive, wouldn't manage that element of surprise. And there is even an achievement which grants you the title 'Warlord of Draenor'
Warlord of Draenor (New) Complete all of the Nemesis quests at the Gladiator's Sanctum and the quest Fight, Kill, Salute! at the Highmaul Coliseum. Title Reward: Warlord of Draenor. 10 points. Account Wide.
So the fifth Episode would be like, when hearing all about those Warlords, Varian asks, 'What hopes do we have then?', and Maraad answers, 'There is one. There are Adventurers, heroes who have gone through the terror of Naxrammas, brought to an end to the mighty Black Temple, defeated the horror of the scourges in Icecrown, made an end to the catastrophic Cataclysm, and brought down the high walls of Orgrimmar'. And when hearing their feat, Varian's face lightens, and say 'We still do have hope then. Now, let's make a plan to fight against the Iron Horde, with their strength combined. With them, we might be able to win this war after all'
And this is the end of my speculation. Hope you like it, and hope this is right. It would be really cool if this really happens