When Thanos clicked his fingers, everything changed. The universe was "rebalanced" just as the Mad Titan had intended, but it probably didn't go as successfully as he had planned. Whether he will even notice that as he congratulates himself in his retirement remains to be seen, but he won't be the one counting the cost.
As the Super Bowl spot for Endgame teased, the starting point for that movie isn't just how the heroes will go about defeating Thanos and reversing the Decimation, it will initially be about what the hell those left behind are supposed to do after a mass genocide event. It's about survivor guilt, about grief and loss and about how that sort of event changes everyone.
This is a different world, now, and everyone left will be completely different. Including all of the heroes and their allies who we know are involved in Endgame's eveents...
12. Tony Stark
At the tail-end of Avengers: Infinity War, Tony Stark is marooned on Titan along with Nebula after the Decimation wiped out the rest of the Guardians and Avengers alliance on that planet.
Up to that point, he's still very much a lead hero, who still runs what remains of the Avengers and still has the same irresistible compulsion to be Iron Man. That continues his thread of not being able to turn his back on his duties, thanks to being "haunted by knowledge" as Thanos so aptly points out. Fundamentally, he proves himself willing to sacrifice himself rather than walk away.
Most importantly, Tony is confronted with two things that will change his mentality for the worse. Firstly, he watches Peter Parker - who amounts to his surrogate son - die in his arms and secondly, he watches the one thing he feared most of all - the universe's decimation - become a reality. No matter what he's done over the years, no matter how much he put on the line, he failed.
We have to consider how that will effect Stark and how it will impact his commitment to remaining a hero (should he survive Endgame). Those two events would have reiterated how hopeless he is, but his tie to Peter will be what convinces him to walk away to be with his own family. Assuming those rumours are to be believed too.
Judging by the Super Bowl trailer, those rumours of a major time jump look to be true and given that Tony is seen to be running out of supplies pretty soon after the Decimation, his biggest change might well be that he dies. That wouldn't be permanent if we're getting time travel, but there's a real possibility he will die in the first act.