One of the things that Hawkeye has traditionally had in the comics, that he's never had in the MCU, is a mask. It turns out Captain America: Civil War very nearly changed that. Some concept art of what Hawkeye could have looked like includes a mask very similar to what the character has worn in Marvel's Ultimate line of comics. Check it out.
The uniform that we see Clint Barton in here is very much like the one that Jeremy Renner ended up wearing in Captain America: Civil War. However, Marvel Studios concept artist Andy Park, also added a mask to Hawkeye originally. The mask isn't just a random addition. It has its basis in the comics, specifically a line that Marvel began in 2000 called Ultimate, which reset the origin stories of all Marvel characters into a more modern day setting. The Ultimate version of Hawkeye had the same mask.
Masks haven't been very important in the MCU up to this point. For the most part, they have existed throughout the history of comics because the heroes tended to keep their identities secret. Nearly everybody in the MCU is open regarding who they are, so masks aren't that important. The one that Captain America wears is a holdover from a WWII propaganda campaign. Iron Man doesn't wear a mask so much as he wears a helmet to protect himself. Same with Ant-Man. Only Spider-Man is actually hiding who he is, and he's the most recent addition to the MCU.
Most would probably consider this a significant improvement over Hawkeye's original costume and mask. Many fans who aren't even necessarily that familiar with the comics would probably recognize that version of Hawkeye more easily. Needless to say, this would probably look a little silly standing next to the rest of the crew in Captain America: Civil War.
It's hard to guess why they decided to go sans mask. Hawkeye hadn't had one previously, so it's possible they just though adding one now wouldn't make much sense. In addition, there's also the fact that Hollywood never likes making their actors unrecognizable. If they're going to pay millions of dollars to put these pretty faces on the screen, they damn well want to get their money's worth.
Would you have liked to have see Ultimate Hawkeye show up in Captain America: Civil War? Let us know your thoughts on the concept art in the comments.