The Burning Throne
Tomb of Sargeras didn't have any special mount drops aside from the Abyss Worm, a special drop from Mistress Sassz'ine that you could get even on LFR difficulty.
Wowhead has pulled some new voice files out of the recent patch 7.3.2 PTR and ... Guys, if we go any further into this I gotta get this spoiler warning out of the way. Because it's big. Spoilers. Teasing the next expansion. You shouldn't read any further unless you want that.
By now, players are unlocking the Netherlight Crucible, the Seat of the Triumvirate 5-player dungeon, and otherwise finishing up with the Shadows of Argus storyline.
Hot off the heels of the world-second kill of Mythic Kil'Jaeden, Blizzard has announced testing will begin for the WoW's next raid, Antorus, the Burning Throne, starting this weekend.
Let me just open this post with one word: Wow.