Just like Pandaria being on the Azeroth globe doodad before it was announced. This expansion holds TONS of hints at what is coming next.
Christ Metzen: "You will need Turalyon and Allerias help for what is next" - Turalyon and Alleria have been gone for years fighting in the twisting nether against the burning legion.
Rob Pardo: "Draeneis and Blood elves will be getting lots of attention next expansion" - Main race focused on the legion.
Besides this, the big one is NPC's that tell you indirectly.
Wrathion: "I believe we are headed towards a reckoning... What Garrosh Hellscream achieved in Theramore is nothing compared to the horrors that are even now bearing down on our fragile home" ... And of course whilst the Wrathion is saying that you see a World (Azeroth) which has the classic "Infernal Landing" Green Meteor bearing down on it.
Zoltan (Master Pet Tamer) is level 47 in Felwood: “Are you ready to accept the Master’s gift? Will you be blessed by his loving wrath, or consumed by it like all of the others? Demons are the future of this world. The sooner you embrace that, the sooner you join the winning team. Sargeras will live once more! All of you must work to aid the great revival of our Master. Only through Him may we usher in eternal peace for all! Our Master, protector of all life, will soon blind all of the heretics and grants us immeasurable power! The Master shall rain hellfire upon Azeroth, and then we will at last cleanse the Light from this wretched world!”
This NPC was introduced in 5.0
Eric Thibeau (stealthed rogue in Shrine of stars): "Northrend still reeks of undeath. Our homelands lay in ruin from the Cataclysm. Pandaria oozes our hatred and doubt. What hope is there for this world when the Burning Legion again lands upon our shores? We openly deal with the ethereal, ignorant of their true intentions. Could we be aiding in the preparation of our own demise"
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Random things introduced in the game as of 5.0/Panda launch
-Sargeras followers all over Felwood
-More demonhunters are present in the game
-The Council of Six Daggers curiously mention two members going back to Outland
I know many people think it means nothing. But blizzard doesn't put TONS of hints in the game for absolutely no reason. It's a pretty safe assumption legion is next, as burning crusade truly was not a legion focused expansion. and the legion is THE major villain in the WC universe now. thoughts?