The first trailer for Avengers: Endgame has confirmed Hawkeye's presence in the movie, but features the Battling Bowman in an outfit that may hint at a terrible fate for his family. The Ronin costume also suggests a new identity for Clint Barton, mimicking the comics that immediately followed the original Civil War event.
Many Avengers fans were upset that Clint "Hawkeye" Barton was completely absent from Infinity War. A throwaway bit of dialogue in the film explained his absence, revealing that both Clint Barton and Scott "Ant-Man" Lang had accepted plea-bargains that allowed them to serve their sentences under house-arrest, after escaping from prison at the end of Captain America: Civil War. With both heroes also being family men, it made sense that they would consider the strain that having a fugitive father would place on their children and chose to put their families first. Given that, it seems more than likely that Thanos's snap - now dubbed The Decimation - resulted in the death of Barton's wife and children.
While there has been nothing so far to confirm this, logically Barton would have little reason to enter the fight against Thanos at this point unless it had become personal. Even ignoring the decimated ranks of the Avengers, there is little Clint Barton could do as one man - but he would almost certainly demand to be a part of any effort to bring Thanos to account - or, at the very least, to restore the lives that were lost. The comic book version of Clint Barton is famed for his temper, and while the Marvel Cinematic Universe movies have not given Jeremy Renner much chance to display this, it still seems likely that the movie version of Barton would demand a chance at vengeance.
The new costume Renner is wearing in the trailer is that of Ronin - a superhero identity used by several characters in Marvel Comics. Clint Barton was the second person to take up the Ronin identity chronologically, donning the costume to hide his identity from the public after he had been killed during the Avengers: Disassembled storyline. He was later resurrected during House of M, when Scarlet Witch used her powers to create a utopia where mutants ruled most of the world and nearly all of Marvel Comics' heroes had been given a happy ending of some kind.
What makes the use of the Ronin costume so interesting is that House of M ended with another event called The Decimation, though that event involved Scarlet Witch depowering most of the mutants on Earth rather than Thanos eliminating half the life in the universe. Both events ended with a high body count, however, and both would leave Clint Barton as a lone warrior with no family and no one to answer to. In short, he would be a Ronin - a samurai with no clan or master to serve.
It is entirely possible that Hawkeye's adoption of the Ronin identity in Avengers: Endgame will differ from the comics. Time will tell the truth of things, but for now it seems as though Clint Barton will be a metaphorically clanless warrior, fighting to avenge his lost family.