Captain America and Iron Man have been two pillars of the MCU since the very beginning. Both of their journeys are likely to end in Avengers: Endgame, partly because it's a finale, and partly because contracts are up. While waiting for more (or any) plot details, fans are filling the void with their own theories. One fan has posted their own intriguing take on what will happen to Cap and Iron Man and how parenthood and sacrifice will play a role.
Tony and Cap may not be speaking to each other at the moment, but one Reddit user thinks that the two of them will be the most important characters in Avengers: Endgame and together, they will cause the death of Thanos.
The Redditor puts a big emphasis on parenthood and sacrifice, which are key to Cap and Tony's character arcs. In the case of Iron Man, family has been a big part of his journey in Phase 3. The death of his parents by Bucky fueled his feud with Cap; he finds a son-figure in Peter Parker; and he wants to have a child with Pepper Potts.
For Cap, family is something that he used to want. He wanted to settle down and start a family with Peggy Carter, but had to sacrifice what he wanted so that everyone else could live. Trading lives is a theme in Avengers: Infinity War and while Cap says they "don't trade lives," it's something that he personally has done and is willing to do again.
As the theory goes, a pregnant Pepper will suit up as Rescue in the final battle against Thanos. Not a great decision on her part, but when the world is about to end, anything goes. At some point in the battle, Thanos is about to lay the death blow on Pepper, but Steve intercepts, sacrificing his life for Pepper and the baby.
This will enrage Tony, who fights Thanos one final time. Tony realizes that he "didn't deserve that sacrifice" and fights to earn it, finally embodying what it means to be a hero. The Redditor points out that Tony wavers between what is right and what is self-beneficial in his own movies, so this will be a true hero moment.
Finally, Tony will wield Cap's shield and will be the one to say "Avengers Assemble," and kill Thanos. Tony will then retire to start a family.
It's a pretty cool theory, when all is said and done. I like emphasizing the parenthood theme that both characters have. Will it actually happen? Probably not, but like any fan theory it sounds cool on paper!
We'll know for sure what happens in Avengers: Endgame when the movie finally releases on April 26, 2019.