The Black Temple is in Shadowmoon Valley. The super-Fel-radioactive place that Kargath and his Horde went into.
With the way the choices and time commitment worked out, we are probably going to end up with Kargath and the Shadowmoon Warlocks as the "major antagonist faction equivalent" for us.
Dealing with Kargath will probably require not raids, but an actual campaign of war.
So basically, what we've taken care of in Nagrand was sort of like the "basic questing" or basic/small battlegrounds or scenarios stuff -- Kargath and Shadowmoon will be, uh, more like Alterac Valley or like the huge collection of Instances that Blackrock Mountain had in Wow.
Conquering and purifying the Black Temple?
One problem is, we don't even know who would currently be holding it... In WoW, it's Illidan's fortress, but Illidan isn't even here. So... maybe the Shadowmoon has it. Maybe Kargath is marching on it to oust the Shadowmoon from it. Or maybe the Dragonmaw were using the Black Temple.
Hell, for all we know, it could be that instead of Hellfire Peninsula, Kargath will just set up in Shadowmoon Valley itself.
My current best-guess is that Kargath went straight for Shadowmoon because he learned about some kind of source of Fel power or a Fel ritual or he heard news of another demon like Mannoroth and wanted to capture it.
I figure that the Shadow Council might have informed him that if he wanted to empower his Horde further, he should head into Shadowmoon. Or maybe they offered to provide Fel energy to Kargath in exchange for him helping them with something. Or maybe they're going to be summoning Mag'theridon and Kargath is either there to assist in exchange for favors... or to crash the party and commit Grand Theft Pit Lord.
... Or maybe he heard about Fel Reavers and decided he wanted some Giant Stompy Robots of his own. Or, uh, he heard about the Dragonmaw and decided he'd head over and "recruit" them in order to get dragonriders...
But yeah... My current thought is that Kargath's journey to Shadowmoon would involve whatever it was that would in canon empower his Horde into Fel Orcs.