Spoiler Warning for people who don't want to know about certain set photo leaks concerning Hawkeye.
Never in my wildest dreams would I have imagined that Hawkeye would become the most talked about character leading up to Avengers: Infinity War, but here we are! Well, it certainly feels like he's the most talked about at any rate. Due to a complete exclusion from all marketing materials for the blockbuster, a mystery has been built from the ground up about the whereabouts of the Avenging Archer. Some people thought that Hawkeye might not even be in the movie, but the Russo Brothers came forward to assure fans that Hawkeye is on a solo mission that keeps him separated from the rest of the crew -- they call it his own "journey" -- and that's it's all connected. Which leaves us to wonder... what is this mission?
As I see it, there are two possibilities for keeping Hawkeye under wraps: The character is doing something so laden with plot spoilers that the marketing can't even show a glimpse of him without spoiling something in the movie; or, Hawkeye barely factors into this movie (but will factor into Avenger 4), and there is legitimately nothing to tease.
I'm an optimist, so I'm inclined to prefer the first option (plus, it's more exciting to think about). So, let's assume that Hawkeye's mission is really important to the end game of the MCU, and that he didn't just retire to his farm to spend quality time with his wife (Linda Cardellini) and kids. There aren't a lot of clues to work with to figure out what this mission could be, but there are some details that have leaked that help to form a foundation at least.
Thanks to photo leaks from the set of either Infinity War or Avengers 4, there's a solid indication the Clint Barton won't just be Hawkeye. Despite the fact that Jeremy Renner was wearing a trench coat, snoopy fans noticed that his boots were extremely similar to the Ronin costume. Ronin is a ninja-like persona that Clint Barton adopts for a time in the comics when he didn't really feel like being Hawkeye anymore. It was a darker time for the Marvel universe as a whole, so it could signal that things won't be very bright and happy in the MCU either.
One potential reason for Clint being Ronin is that he needs a new identity to do whatever mission he needs to do. Hawkeye has a pretty recognizable face, so wearing a mask could be a way for him to operate with some secrecy. Clint was last seen getting broken out of jail by Captain America, so it makes sense that it's Cap who sends him on this mission in the first place. We know that Cap has been busy protecting people from the shadows since the events of Civil War, so any mission he'd give to Hawkeye could be related to that goal.
When it comes to mysteries in Infinity War, most people's minds would jump straight to the Soul Stone, the final piece of the Infinity Gauntlet. The stone is still MIA, so what if the missing MacGuffin is related to the missing Hawkeye? One internet theory is that Hawkeye's mission involves the Soul Stone in some way, either tracking it down, guarding it, or something else. It certainly kills two birds with one stone, but then how does Hawkeye even know about the Soul Stone in the first place? If we continue with the idea that Cap sends Hawkeye on a mission, then it doesn't really make sense. The trailers don't indicate that Cap is involved with the hunt for the stones and that he's instead busy doing Earth stuff. Plus, if the Soul Stone is in space -- as one popular theory suggests -- how could Hawkeye, of all characters, possibly find it?
It's also possible that Hawkeye is paired off with one of the other missing Avengers: Ant-Man (Paul Rudd). Ant-Man has likewise been a no-show in the marketing material for this summer's movie, so could the two of them be on a mission together before receiving bigger roles in Avengers 4? One theory claims the Soul Stone could be hidden in the Quantum Realm, which we know Ant-Man will be journeying to in his sequel. There's a chance that he goes to the Quantum Realm again to get the stone, and he brings Hawkeye along as backup. There's no strong evidence to support that, but I think people like the duo enough to see them perform more sweet combo moves together.
Tying back into the Ronin persona, there could be a potentially big thing Hawkeye could be doing that would connect to Avengers 4 or even Phase 4. It's perhaps notable that one of the biggest storylines from Clint's time as Ronin was Secret Invasion, during which Earth's heroes realized that the Skrull had covertly infiltrated the planet and replaced key figures with sleeper agents. Ronin was on the Avengers team that learned about the invasion, so perhaps Cap sends him on a mission to check out some sketchy stuff, only for Clint to stumble upon the Skrull, learn their plan, and then go dark because he has no idea who he can trust. This could be the plot for Avengers 4, but I'm thinking that the capper to Phase 3 will have to be Thanos related.
It's pretty cool that people are this obsessed over Hawkeye, a character who's rarely ever been the star of the show. The biggest fear about his lack of presence is that the mystery is getting blown out of proportion and it won't live up to the hype people have built. That's kind of the biggest fear with the entire movie though, but here's hoping that the Russo Brother and the team can pull it all together!