In the comic book storyline Civil War, Iron Man (Tony Stark) does some horrible things. Then Spiderman made a deal with the Devil to reset that to make it so that Iron Man is a good guy again. But the movies can't be reset so easily, so how can he be redeemed?!Don't Comment Captain America: Civil War Spoilers, please!
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If the Civil war movie follows the Comics then Iron Man will do lots of bad things. I mean, more bad things. So Tony Stark can't be in The Avengers if he has done more harm than good. So how can he be redeemed?
He Can defeat Thanos!
After all the Avengers hate Tony Stark because of his devious acts he can save Them from Thanos evil Grasp. It can also save him from his PTSD that he has, or had.
It could be done with a loose interpretation of the Demon in a Bottle story!
It could go something like this: After Civil War, Tony Stark gets kicked out of the Avengers. Then he gets into heavy drinking and we see it in Avengers: Infinity War part 1. He sees his team getting beaten up by Thanos. Then he steps in with some gadgets or armor to stop Thanos.