No, seriously, spoilers! Also spoilers for Captain Marvel.
What if all of Endgame was just a dream?
Tony is stranded on the Benatar at the beginning of the film, before he starts to "drift off." What if he actually did drift off into a comatose state?
A lot of things in Endgame didn't make sense to me. I was a little let down by the movie's cop-out of time travel to solve the dilemma. Especially confusing was the characterization of 2014 Thanos. When he (AND IF YOU'RE STILL READING THIS WITHOUT SEEING THE MOVIE, PLEASE STOP) tries to snap in the "eternity war" at the end of the film, he was going to destroy absolutely all life, not just half. That doesn't fit his characterization. Ultron was the one who wanted to wipe out humanity, and that was because Ultron is not a logical being. Although both he and Thanos are motivated by the mind stone, Thanos pursues the more logical approach of keeping half the universe. Where is his motivation to destroy it all? Doing so would place him in the position of god, which he doesn't want. His throne room shows that he's not looking for glory, just a solution to the problem of life.
Tony doesn't know this. All he knows is that Thanos is genocidal, trying to destroy New York, not cut it in half. He would imagine Thanos doing something like what almost happened at the end of this film.
Furthermore, Tony doesn't know what's going on with Bruce and Hulk. All he knows is that they're having "a thing." He could imagine Bruce and Hulk being in the process of reconciling their differences (at the beginning of Infinity War), but that wasn't the situation. Bruce had only just become aware of the Hulk's reluctance to emerge; bringing Professor Hulk into the first act of Endgame seems a little too quick. However, it makes sense if Tony thinks that Bruce has been wrestling with Hulk since Age of Ultron; he doesn't know about Hulk's time on Sakaar.
And the whole time travel plot seemed
way too quickly introduced into the movie. Scott's theory doesn't even make full sense, but it's just a hypothesis. However, this hypothesis is very quickly confirmed to work. And the rules of time travel are said once.
Once, by Hulk.
So if Tony was dreaming, then Thanos' ultimate genocide plan makes sense, Professor Hulk has longer to have been developed, and time travel makes sense because Tony's imagining it and making up the rules as he goes.
However, how is Tony getting the extra information he needs for this dream? Tony had a dream in Infinity War about a child, named after an "eccentric uncle" (if I remember correctly). He already had that information prior to the dream, simply imagining the events in his dream based on what he already knew. But if Endgame was Tony's dream, how does he know about Captain Marvel, the Quantum Realm, or Korg and Miek (among other things)?
Remember in Age of Ultron, when Scarlet Witch gave Thor his vision of the Infinity Stones and Vision? Scarlet Witch could not have known about these two things before she gave him the vision, nor could Thor have, but they still appeared in the dream! Scarlet Witch's telepathic abilities were able to give Thor this vision. It wasn't a vision of every event about to take place, but key details from the upcoming event. If Endgame was a telepathic dream given to Tony, then he would be able to see Captain Marvel (the "strongest" [yeah, right] Avenger), Ant-Man (the smallest Avenger) and his current location (the Quantum Realm), and Korg and Miek (survivors of the attack on Asgard).
But I know what you're thinking. Scarlet Witch was decimated by Thanos. If Tony's dream starts at the beginning of Endgame, then she's already gone. Who would give Tony such a dream?
Captain Marvel. She has telepathy. See this clip, around 49 or 50 seconds in. Listen closely:
https://youtu.be/9QbltzIUV6w?t=49
After the mid-credits scene of Captain Marvel, when she arrives at Avengers HQ, she goes to look for Tony. Why does it take her so long (22? days) to find him? What if it took even longer to find him, and once she did, he was almost dead? So what if she gave Tony the beginning of a telepathic dream, to try to wake him up, and Tony filled in the rest with his ideal end of his life?
I'll let my theory rest for now.