Captain America
Let’s face it Cap may not give up the fight until he has to. So, after the battle with Thanos he could find a way to take away his powers. He could then hand off his uniform to either Bucky or Sam. This way he will not feel as guilty about losing his powers because he is leaving things in the hands of one of his best friends. Finally, Cap could walk off into the sunset with Agent Sharon Carter and get the normal, family life that he deserves.
Thor
After helping to heroically beat Thanos, Thor could go to find the people that are left of Asgard. Let’s face it not all them were killed by Thanos and his men. I mean why would they introduce Valkyrie just to kill her off screen? Maybe Loki will even be with her. Anyways, Thor could be shown bringing his people to the same place his father died in Thor Ragnarok. This is Norway.
Iron Man
After the end of the infinity wars, Tony could be shown reuniting with Pepper Pots. She could announce that she is pregnant. This may prompt Tony to finally retire from his days of being a hero. This way he will not have to worry about risking his life and can be there for his child. Before leaving he may even hand off the suits The kid he met in Iron Man 3, Harley Keener. Maybe after that they could even do a flash forward in the movie to show Tony working on inventing things with a son named TJ.
Hawkeye
After the war, Hawkeye could finally retire and go to be with his family. He could be seen teaching his kids to use a bow and arrow. Rumor has it that his daughter is supposed to become a hero, so maybe his story could end by handing off his bow and arrow to her. So, that one day she could use it for her heroic adventures.