While Avengers: Infinity War was an epic film, it did lack in one main area. Several of fans' favorite characters barely got any screentime. Black Panther, Captain America, Black Widow, and several others were relegated to a page of script. The only heroes that really had prominent screentime were Gamora, Iron-Man, Thor, Doctor Strange, and Spiderman. While it's obvious that crunching dozens of heroes into one film means that there will be sacrifices, no one expected the Avengers leader Captain America to be reduced to only a few lines.
Screenwriters Christopher Markus and Stephen McFeely sat down with The New York Times, and explained that everything happened for a reason and that the cliffhanger wasn't always planned that way. “It wasn’t just, well, we’ve got too much story, we’d better chop it in half,” stated Markus. The writers reveal that "the snap" was intended to be in the fourth film, but they ended up putting it in the third because, "it would feel more like a cliffhanger than we intended," if they left the quest for the stones incomplete.
The writers also explained that Marvel and Disney didn't mandate any specific story arc for characters, allowing them to create the team-ups and screentime that fans saw as the final product. Markus stated that, “we’re never writing without knowing where the end goes.” When it comes to Captain America and Black Widow he revealed that they, “have a much bigger role to play in that second film.” McFeely added, “We gave ourselves license to pay off later.”
That forward thinking and ultimate "payoff" has also been discussed by Marvel Studio head Kevin Feige who promises that characters like Hawkeye and Ant-Man have important stories in the long run as well.