Marvel hasn't revealed much about the plot of Avengers: Endgame, but the two teasers for the film have both shown one major development: Steve Rogers is a desperate man. It's a side of Captain America we've never seen before, one that looks to be setting up a major role reversal for him and Tony Stark. While that might not be a good thing for Cap, it might be the best thing that can happen to the Avengers.
The movie's trailer and new commercial have both featured a Steve Rogers we don't recognize. Instead of the determined soldier who fully commits to a course of action, he's an emotional wreck without purpose. The trailer features him sitting alone with a tear running down his face, and then when Natasha tells him their plan is going to work, he says, "I know it is, 'cause I don't know what I'm gonna do if it doesn't." Those aren't the words of a confident man, they are the fears of someone who can't handle the idea of failing again.
He is downright stable in the trailer compared to how he is in the commercial though, in which he's at a group therapy meeting for survivors. He sits, unsure of himself, uneasy, like he would crawl out of his skin if he could. The Avengers lost, he let down his friends and the entire universe, and he doesn't know how to accept or process any of it. Captain America has always faced the prospect of losing bravely, like in Age of Ultron when he told a worried Tony Stark they'd "lose together," but he's never actually lost until now. The Great Dusting has left the normally unflappable hero a mess. It has turned him into a truly desperate man who would go to any lengths to undo this terrible thing he blames himself for. The Snapture has left him looking a lot more like Tony Stark.
Ever since Loki brought any army to New York City through a portal in space, Tony has been driven by one fear: that it all might happen again. That sole focus has led Tony to make a series of rash, emotional decisions, all of which have been catastrophic. Ultron, Tony losing his mind and building a massive fleet of new suits (twice), the fracturing of the Avengers over the Sakovia Accords (further attempts at controlling the uncontrollable)–all of that happened because Tony was fighting a war before it happened. Despite Tony's genius, his desperation to save the world and his loved ones causes him to act with his heart and not his head.
And yet, in the little we've seen of him in the Endgame footage, he's been decidedly un-Tony. When he was dying in Iron Man 2 he was distraught and self-destructing, but in the Endgame trailer, he seems at peace with his impending death. And in his brief cameo in the commercial, he's hard at work with Nebula, trying to resolve a problem. We don't know the exact context of that moment, but combined with his calm confessional in the trailer, a picture is developing of a more even-keeled Tony who is dealing with the issues in front of him directly and rationally. He's dealing with them the way the Steve Rogers normally does, all while Captain America is acting more like the typically emotional Tony.
It's a complete role reversal for either, and while it should have Captain America fans worried for his safety, this development might ultimately heal their friendship and save the universe. The Avengers lost to Thanos because they weren't unified, and the heart of that split was the demise of Steve and Tony's relationship. Now though, Steve understands what has always driven Tony, because for the first time in his life he's experiencing the desperation that makes Tony tick, for better and worse. And Tony will know exactly how Cap feels, because that's how he always feels. The two heroes will be able to empathize with each other in a way almost no one else in the world can, which will strengthen their relationship. And unity is the only chance they have for undoing the Snap.