This post contains major spoilers for Avengers: Infinity War. There's no denying that Avengers: Infinity War left fans utterly heartbroken. But the truly sad part is things could get a whole lot worse for Earth's mightiest heroes, especially if this Avengers 4 fan theory turns out to be true. At the end of the film, half of the heroes (along with much of the general population) were killed by Thanos. This is in addition to the deaths of key characters like Gamora, Loki, and Vision earlier in the movie. It's pretty obvious that most of these deaths won't stick — Black Panther and Spider-Man have sequels they need to be alive and well for, after all. And that's why, truth be told, Avengers: Infinity War's carnage isn't too upsetting. What fans should really be afraid of is what comes next.
BuzzFeed writer Keely Flaherty has spun an Avengers 4 theory that brings the devastation home for the franchise's core heroes in a meaningful and long-lasting way. While much of the action in the next movie may hinge on the use of the Time Stone to tinker with the timeline — likely with the help of Captain Marvel and Ant-Man — Flaherty posits that it's the Soul Stone that fans should be focused on. Sadly, time travel isn't going to solve all of the Avengers' problems.
When Doctor Strange looked into the Time Stone, he examined millions of futures, and he only found one where the good guys win. He seemingly believes that he's set that future in motion given that his last line is, "There was no other way." He likely knows that this large sacrifice won't stick, and instead, what the world needs now is a singular sacrifice that will take the MCU back to its roots.
Flaherty's theory hinges on the idea that the remaining Avengers need to get their hands on all of the Infinity Stones in order to defeat Thanos for good. However, that means that the Avengers will have to possess and be able to wield the Soul Stone, as well as the others, and that requires a sacrifice — one that's been a long time coming.
Remember, Thanos had to kill Gamora in order to gain control of the pivotal stone. It requires the owner to give up someone that they love. The Avengers have proven time and time again that they don't believe that there is such a thing as an acceptable loss. Captain America even tells Vision that "they don't trade lives," but in this inevitable future that Doctor Strange saw, they may have to. And you better believe that if it comes to that, then the group's two most stubborn heroes will be the first to put their lives on the line.
Tony Stark and Steve Rogers have a complicated relationship at the best of times. They are fundamentally different people, who have been butting heads since the day they met. However, their goals are always the same — even if they sometimes disagree on the road that needs to be taken in order to achieve them. The one thing these two men want more than anything else is to protect the people they love and to keep the world safe. In fact, they've already both sacrificed themselves for the greater good before, and they could be preparing to do it again.
If the key to possessing all of the Infinity Stones lies in a willingness to sacrifice someone you love, that would be a problem for the Avengers. Tony isn't going to kill Pepper, Cap wouldn't give up Bucky again, and Hawkeye certainly isn't going to kill his wife. These are not acceptable terms. But would Steve and Tony be able to sacrifice each other? The answer to that question is almost certainly yes.
Here's the thing, Tony and Steve respect each other. They understand that deep down they want the same things. And yes, there is love there. They're comrades who understand the burden of carrying the weight of the world on their shoulders better than anyone else. They also likely know that either would give their life if it meant the people they love get to keep on living.
Given that this sprawling franchise began with Iron Man, it seems only fitting that this phase of the MCU would end with him giving up everything so that the rest of the heroes could go on. Tony has felt like he's been living on borrowed time for quite a while now. He may believe that giving Steve his blessing to sacrifice him for the greater good is the only way, and it would be a huge moment for both of them. Steve would have to admit that sometimes you do have to trade lives, while Tony would give up his last chance at hanging up his suit and living a normal life with Pepper.
Or the opposite could happen and good old Steve could once again show his selflessness by forcing Tony to take his life. Either way, it would be devastating and rooted in the deep history of the MCU up to this point. Most importantly, it would be a permanent loss, and it would signal the beginning of a new era for the franchise. And seeing either Tony or Steve die would hurt. Oh, would it ever hurt.