Wrathion was supposed to be a key player in the return of the Legion Expansion. He had been preparing the Horde and the Alliance for the eventual grudge match between Azeroth and the Legion since Mists of Panaria. Then, he just fell off the face of the world in Warlords of Draenor. Either he was neglected, or pushed aside to make room for the return of Illidan and Turalyon, not to mention Velen and Khadgar would also be key players in the fight against the Legion, so suffice it to say, Wrathion's role either went to Illidan or Turalyon and Blizzard felt Wrathion had no place in the Legion story with an already crowded cast.
He has no stakes in Battle for Azeroth, much less a rumored Wrath 2/Undeath expansion, so what if he's being groomed as WoW's next big original villain, much as Garrosh was? With a sect of the player base asking for an old world revamp like Cataclysm, could we see a Cataclysm 2 featuring Wrathion finding the threat of the Void Lords insurmountable? Maybe he decides to finish his father's work, not necessarily destroying the planet, but wiping out life on it to spare it becoming tainted by the whispers of the Void? The emergence of Void Elves certainly would paint a world at the brink of madness to Wrathion, especially with Sylvanas' foolish war, maybe the mortal races of Azeroth aren't worth saving after all, nor deserve it, the Horde especially since their main concern is weaponizing Azeroth (the one being trying to heal the planet is a former Alliance King.)
Not to mention, being mortal himself, Wrathion might want to avoid going down the same path of madness his father did, so rather having to fight the Void himself, just wipe all life on the planet out himself. Does he have the power to do it? Maybe not, but power is something that can be attained at the flip of a dime to make him a formidable foe to both factions (see Garrosh drinking an Old God heart smoothie.) He's certainly intelligent enough to challenge both the Alliance and Horde and a cunning, mental adversary opposed to a raging beast would be a refreshing enemy to take on, as somewhat of a redemption to Deathwing's portrayal in Cataclysm. Maybe use his friendship to Anduin to infiltrate Stormwind and manipulate the Alliance through Anduin.
A Wrathion-centered expansion could also update the status of the Dragonflights and maybe even introduce the much talked about Dragon Islands.
What do you think Wrathion is up to and how would you like to see him re-introduced?