In the first 3 campaigns of World of Warcraft - classic, Burning Crusade, and Wrath of the Lich King, but the overarching plot of all three campaigns was Arthas and the Frozen Throne.
In the background/leadup/as a consequence of the overarching plot, each expansion had a Minor enemy force to Major antagonist in Arthas.
In Classic, it was the Silithid.
In Burning Crusade, despite being the 'final boss', Kael'thas and the Sunwell were secondary elements to Outlands and Arthas' trail of carnage.
In Wrath, it was the Titans and Ulduar - important but underlying to the Scourge and Icecrown.
These new expacs? Are a rehash of those secondary elements, brought to the fore.
In War Within, it's the Nerubians - different to the Silithid, but... Very... Very similar. (Also they're probably under Silithus b/c of Sargeras' sword, etc etc)
In Midnight, it's void entities attacking the Sunwell in Quel'Thalas - again, everything not-Arthas (and not Illidan in this case) brought to the fore.
In The Last Titan, it's Ulduar and the return of the Titans, and the conspiracy surrounding the Titans' true intentions.
Finaaaaalllyyyyyy.
If nothing else, I'll say this: Metzen has his flaws, I will not deny or excuse them. I AM glad to see this level of commitment and planning to a story! Actually makes me somewhat excited, as a Solo-Singleplayer-Story-only kinda guy!