Though it hasn't been confirmed yet, it's widely expected that Avengers 4 will see the arrival of Hawkeye's darker, edgier alter-ego Ronin when he returns to active hero duty. The suspicion is that something will have driven him to have give up retirement AND go all emo and the only thing that fits (given his backstory established in Age Of Ultron) is the removal of his family.
After all, he (and Scott Lang) took a deal in the wake of Civil War and accepted house arrest for the sake of his family (literally almost a direct quote that). So why would he willingly pull out his bow again if there hadn't been a big change in his circumstances?
Worringly, that suggests the Bartons may have been victims of Thanos' snap at the end of Infinity War.
And there was actually a bit of foreshadowing to that end from a strange source: the Japanese posters for Age Of Ultron, where we first met the Barton family. Despite the fact that there is no plot detail relating to Hawkeye protecting his family (other than a vague allusion to him retiring for their benefit), he's pictured fiercely holding them close to him from some sort of threat (while looking like he's just remembered he left the oven on).
It's not the best image in the world (and Linda Cardellini looks like she's laughing for some reason), but it offers a huge hint at why Hawkeye would come back into the fold and why he'd have a darker character in Avengers 4.
Here's the full thing in all its delightfully odd glory...
As an added bonus, here's the alternate poster showing Bruce Banner and Natasha Romanoff. At least, I THINK that's who those people are. Banner's hair is both astonishing and utterly compelling. Try not to stare at it, I dare you.