I don't post often bare with me. One version of civil war ends with steve dying I feel Tony doesn't realize his actions until they lead to someones death his foolishness leads to steve's death. I wouldn't know how to write him till infinity war part 2 but i had this idea. Tony, Hulk, the Guardians, Captain Marvel, whoever is fighting Thanos.
I feel time is the most powerful gem and they can not guard against it so they devise a plan to remove it from the gauntlet. I don't see how you can win if you control time on your side. They finally get it removed. I must emphasize that steve is dead and bucky the winter soldier has taken up his legacy. Everyone is estastic but tony immediately goes for it not to beat thanos but he wants to correct steve's death more than anything. He grabs it and goes to a earlier scene seen in civil war. He spills his soul to steve telling him he was wrong and wanting to reach an agreement. Filling satisfied he goes back to the present fighting thanos.
Hoping to see steve he doesn't it is still Bucky and he is disheartened. They still unearth the time gem from the gauntlet cause its overpowered and asks why his actions didn't matter. They did however, the original timeline ended with a massive distrust of heroes and terrible riots and misunderstandings. Tony going back in time created a new timeline where the original distrust was combated by both of them leading to a new trust in heroes. The original timeline would be heroes thinking humanity doesn't know anything we gotta beat Thanos, The new one would be humanity still trusts us they sent us out there.
I say this next part because this is not comics but building on the cinematic universe. Of course cap was worthy for mjolnir and that scene from the comics is amazing. It will be upsetting to steal an amazing moment from the comics but what if tony after realizing how he was wrong and seeing how he has no control but wanting to save his friends gives it all up and becomes worthy.
We expect cap of course he would be worthy. But lets change comics for a moment tony was terrible in civil war and in the movies. Lets let him be completely changed and inspiried. Let him close and die in this infinity war movie. He finally only cares about fixing wrong, giving up his pride and looking out for himself helping his friends and letting go of his guilt. If that isn't worthy then whats the point. Tony Stark picks up mjolnir and lets Thanos have it he first tries with the iron man armor of course it doesnt work. He doesn't care anymore and picks it up himself. Insert scenes he lets Thanos have it. He dies because a human and a hammer can't beat a titan and a all powerful glove but he gave it his all. He dies Captain Marvell and the Guardians defeat him and he dies a hero that wasn;t but realized a real hero is who gives his all.