As has already been explored in the MCU, characters being forced to watch their friends and loved ones die (or almost die) tends to have a pretty big impact on them. Tony Stark was fundamentally changed when Happy and Pepper were almost killed in Iron Man 3, Cap was changed by Bucky's "death", the Twins were twisted into villains by the death of their parents and there wouldn't be an Avengers team at all without Phil Coulson's death.
So it seems inevitable that the aftermath of Thanos' devastating snap will lead to lots of changes among the survivors. They've just watched their allies fall in close quarters, they've lost loved ones and people who they wanted to protect more than anything. They simply cannot be the same.
And to that end, the Russos have confirmed that two characters have been fundamentally changed. As part of their feature on every Infinity War character, the directors talked about the Secret Avengers, who they say were a far more hardened group than the last time we saw them. Having been working underground on missions in secret, the core team of Steve Rogers, Sam Wilson and Natasha Romanoff have grown tighter and edgier. They said as much when talking about Black Widow in particular:
“In Winter Soldier, what we found so compelling about pairing her up with Captain America was that he is this paragon of morality, and she is a paragon of gray. But they’ve grown together, and I think when you find them in [Infinity War], they’ve been the Secret Avengers for two years… they’re like a tight-knit family now. Her, Sam Wilson, and Steve Rogers. It’s hardened all of them in a way.”
But it doesn't end there. After Falcon became one of the heroes to die when Thanos clicked his fingers, the two survivors went even further down that development path:
"I think all of those characters, specifically the ones who survived Infinity War — Cap and Natasha — we will see them with a much harder edge.”
By the sound of it, we're going to see far edgier versions of the Avengers when Avengers 4 rolls back around - which goes some way to explaining why Hawkeye will be returning as Ronin rather than in his usual guise. Global catastrophes have a habit of changing you, after all.