This post is full of spoilers. Come back after you've seen Avengers: Infinity War and we'll talk some more.
The end of Avengers: Infinity War messed us all up, sure. But we seem to be in agreement that most of our many faves who died will somehow be resurrected in 2019's untitled fourth Avengers movie.
Of course, there's the Time Stone. It might be enough to reverse the mass murder Thanos committed just by getting the Time Stone back and hopping back in time a bit. But that also might not be possible.
It can't all rest on the Time Stone, that's too simple. So, I have a theory. And you're not going to like it. The Infinity Stones are mysterious, and we don't know all the rules that come with a single person possessing them.
Maybe just getting the Time Stone back won't be enough. Maybe because Thanos used all the Infinity Stones to kill half the universe, all the Infinity Stones will have to be possessed to reverse the action. Plus, the only way they'll ever defeat Thanos is by gaining back all the Stones — he soundly defeated the Hulk with just one.
And we know that there's one Stone that's particularly hard to gain possession of: the Soul Stone.
If the surviving Avengers need to repossess all of the Infinity Stones (including the Soul Stone) to undo Thanos' actions, one of them will have to kill someone they love.
Basically what I'm saying is that there are some unbearably heartbreaking scenarios we could see play out between the surviving heroes. Natasha could be forced to kill Bruce Banner.
Tony could murder Pepper (if she survived Thanos' attack).
Any single surviving Wakandan could die and that would not be OK.
Or, maybe worst of all, either Tony or Steve Rogers could be forced to kill the other. They both survived Infinity War, they're gonna reunite to stop Thanos, and there's no way we get a happy ending.
We've pretty firmly established that if Avengers 4 brings back all the major players killed in Thanos' attack (which is likely), some people will have to stay dead (Loki, Heimdall, maybe Gamora), and one of the central Avengers will probably have to die, too.
Cap and Tony are the most likely contenders. Tony's been having that horrific death vision since Age of Ultron, and Thanos even told him in this movie that he knew him, and that Tony isn't "the only one burdened with knowledge."
They're the two heroes that have been around the longest. They have plenty of solo movies. And it makes narrative sense. How heartbreaking and awful (but also brilliant) would it be to have them finally and truly reconcile after the events of Civil War and Infinity War, only to make the ultimate sacrifice for each other?
But think about it. The Soul Stone sacrifice would prove that despite their longstanding rivalry and difference in opinions, the two superheroes did love each other in their own ways.