In a worst case scenario, it could easily be the end result of the titular team’s confrontation with Thanos in The Avengers: Infinity War.
Of course, the big one is the hallucination that Tony Stark is given in Baron Strucker’s hidden base just before the title card in The Avengers: Age of Ultron.
Meanwhile, a portal has opened up in the distance, with a full herd of Chitauri Leviathans flying towards Earth.