Seriously. The ideal way they could have pulled this off is by conjuring an entire alternate Draenor a la the Timeless Isle, from one "fake" reality into our own. So, when Gul'Dan would have contacted Kil'Jaedin, it would have been MU KJ, and there would be no AU. It would still be interesting as hell, giving significant implications to the legion copying themselves.
There is literally no possible satisfying end to Warlords of Draenor as it stands, though. We now know there is an infinite number of alternate universes, all with their own Burning Legions, and they can communicate with one another and allow for travel between those universes. There isn't just one Burning Legion, but countless Legions, and Sargeras(es) (is/are)n't stupid. The only possible way we can end this is by cutting ourself of from every other timeline, permanently. But, we've already been to this alternate universe, and there are already AU Blackrock orcs in the Blasted Lands. Would they disappear in the MU continuity, but remain in-game like the fightable Kor'Kron in the Barrens?
Where can we possibly go from here, aside from severing all ties with all other alternate realities?