There might be an ulterior motive for keeping Iron Man aka Tony Stark alive at the end of Avengers: Infinity War. A fan theory explains that Stark might be the ultimate sacrifice for the Avengers to defeat Thanos and save the world.
In the Avengers: Infinity War film, Thanos snaps his fingers causing half the human population and half the superheroes to vaporize. The Avengers will have to look for nothing less than a miracle to reverse the events. To do so, we can expect time travel to play an important role. We also have our new Marvel superhero, Captain Marvel for the rescue. But this might not be enough to undo the catastrophe.
A fan posted on Reddit about his crazy theory that explains, Stark will have to sacrifice himself in order to bring his friends back.
The theory reads as;
Avengers 4 will very likely take place in the future, at least for the first act. The evidence is piling up like an older Cassie Lang casting, grey-haired Tony, Black Widow's hair in the promo art is much longer than in IW (that is at least two years of hair growth), etc.
There has been a casting call for a baby with dark hair. It is listed as a special role. Most likely Tony and Pepper's son.
Post-Infinity War, Tony has settled down with Pepper and they have a son. Soon thereafter, Ant-Man shows up and time travel is introduced, so now Tony hatches a plan to reverse the snap... However, this means forfeiting the current timeline where he has his son.
After they arrive on Vormir with direction from Nebula, they will be told a sacrifice needs to be made. It was introduced Red Skull can see everyone's soul as the Stonekeeper when he addressed Thanos as "Son of Alars", so it is possible he will peer into Tony's soul and learn he is in a harrowing quandary: sacrificing the timeline where he has his son to save half the universe. As the Stonekeeper explains this, a tear rolls down Tony's cheek.
Maybe this will qualify as a sufficient sacrifice to claim the soul stone?
The discussion roared on Reddit with people commenting about Stark naming his baby boy as Peter in reference to Peter Parker aka Spider-Man.
"Ah damn, and they're going to name the baby "Peter". He'll have to lose him twice."