The last boss of Hellfire Citadel is Archimonde. I really doubt he will be the last boss in WoD, so... another raid tier?
And what's going on with Gul'dan and Grom? They are not bosses in HFC.
Also that reanimated Mannoroth... Grom killed him 2 times, we've defeated him one time in WoE. So again? It's just meh.
inb4 Reanimated Reanimated Mannoroth.