Expansions are prolly going to be around one of these ideas I would guess.

Emerald Dream and the Old Gods. We have Yesera the Green Aspect out and about now. The Stormrage book and the fact that the portals to the Emerald Dream have been in the game since vanilla.

Burning Legion: Sagerous hasn't been touched, we have underwater zones and the maelstrom now. Lady Azshara is untouched lore still.

Twilight Dragonflight. Been fighting these guys for 3 expansions, Knaak's books support this as a possibility. Nosdromu was present during Thrall's wedding thing. As far as I know thats the only time he has ever been "present' for something. The graveyard Nosdromu in lich king doesn't count. Might bundle this in with an Emerald Dream expansion as its all old god stuff as well.

New worlds: There are those heroes from the warcraft RTS games that went through the portal and never heard of again. The ones that have the statues at the entrance of Stormwind.

My guess is they have the next 3 expansions on the drawing board if not more and most likely had BC and LK on the drawing board during the launch of vanilla.