Hmm.. long shot, but reasonable..
I don't think THAT much time has passed since Ant-Man. This makes me lean more towards Avengers 4 being loosely based on Avengers Forever. The Destiny Force will be replaced by the Quantum Realm. The Avengers will either all be killed or they'll be scattered throughout time and reality without their memories. That's probably where the Quantum Realm comes into play. They'll use it to travel between realities to locate the scattered Avengers, or if they have all died in the fight against Thanos, they'll look and find the best versions of the Avengers to unite and bring back and fight Thanos in the present/past, like in Avengers Forever. The BARF technology will be used to show them events from their actual reality. Showing them that they're a team and how things came to be after their encounter with Thanos.
That's why in the leaked photos we see things like Fury as civilian and Tony in a SHIELD jacket with blonde hair. In the photos you'll also see Tony and scumbag Steve wearing portable wrist variations of the device that's embedded in Scott's suit.
Most importantly is the aftermath of Avengers Forever. A consequence of the fight is that the alternate versions of the Avengers formed in an alternate past/reality now exists in mainstream reality. That makes it possible that Ms. Marvel, Fantastic 4, X-MEN and are all able to exist in the new present time. I know we're getting a Captain Marvel movie before Avengers 4, and while that movie shows her existence, A4 will show how she's brought back through the Quantum Realm. She'll be found in the Quantum Realm during their journey to find the Avengers.
