One of the finer things about Captain America: The Winter Soldier is that it was primarily S.H.I.E.L.D.-centric, showcasing street level superheroes and villains that allowed it to take on a more realistic action tone, as opposed to sci-fi. This meant that Black Widow was able to appear without turning into a deus ex machina (something that the Hulk, Thor, or even Iron Man would turn into if they helped Steve in the film.)
The 3rd film in the series will most likely follow the same tone, if we’re going by what Jeremy Renner has hinted at during a statement made to Collider. According to Renner, Hawkeye could make an appearance but he’s not exactly sure what’s going to happen:
“There are rumblings of Captain America 3,” says Renner. “I don’t know what’s going to happen there. I’ve got to find the time, man! I’m trying to stay home and I can’t.”
Hawkeye is a good fit for the Captain America film, for the same reasons as Black Widow’s – namely that he doesn’t have any superpowers. Additionally, it also teases the rumor regarding Captain America’s demise in the 3rd film, which was spurred on by Chris Evans’ contract nearing its end in contrast to Sebastian Stan’s being extended: when Steve Rogers was assassinated in the comic books, he was replaced by Clint Barton (having proven himself to be one of the few heroes who could use cap’s shield effectively) briefly, before dropping out. The role of Captain America then went to the newly reformed Winter Soldier, Bucky Barnes.