Honestly, there shouldn't be a "thing" that gets the Horde/Alliance fighting again. What it should be is a series of small events that gets the factions upset... and continue that way from now until the servers shut down decades from now.
I personally feel the faction war serves faaaaar better as the special spice to every expansion story... and fails EVERY time that spice gets forced to become the primary story! There's no point in making the faction war THE focus of the story, because NOTHING ever gets solved, and you're always going to leave 1/2 of the audience dissapointed by definition.
And for those bullying doinks who shout "But it's the focus of Warcraft since the RTS!", and I'm here to remind you that - NO, not even by the end of the RTS was the faction war the core focus anymore! Even Blizzard (well, OLD Blizzard) learned that by Warcraft III when they made the LEGION the primary villain of that story, and let the faction war simply be the icing on the cake where fans of all factions got development, both good and bad.
Seriously, stop making faction leaders into heroes and villains and go back to having the feud just be something extra that happens in the middle of the story, with maybe some new characters comming out and being developed that aren't faction leaders, but generals and tribal chieftains who get their own story that wraps up by expansion end.
Let the new major villains arise that we both fight, and let that fight bring out the good and bad in our faction disputes.