Each expansion used to have some kind of troll content. Vanilla with ZG, TBC with ZA, WotLK... well there was that whole troll zone I guess. Cataclysm had the revamps of the dungeons, MoP had the isle of thunder (though kinda split with the mogu). Haven't had anything since then so wouldn't surprise me they are taking a break from those things until releasing some very big patch where Zul himself fits in.