Warning: SPOILERS ahead for Avengers: Endgame.
Black Widow and Hawkeye's mission in Avengers: Endgame is a major one, but an earlier version of the film's script called for Thanos' forces to interfere and up the ante. Neither Natasha Romanoff (Scarlett Johansson) or Clint Barton (Jeremy Renner) have received solo projects to this point in the Marvel Cinematic Universe, but whenever they do appear, the movies normally show just how close their friendship is.
They survived Budapest together, but their obtain the Soul Stone, and even after agreeing that it had to be done, they didn't agree on who should be the one to go. The two trained fighters tried to settle the difference that way, and it was ultimately Black Widow who "won" and was the one to sacrifice her life.
The scene is incredibly personal in Avengers: Endgame, but co-director Joe Russo revealed that this wasn't always the case. During the scripting phase of the film, Joe revealed on the Happy Sad Confused Podcast that an earlier version of the scene featured Thanos' forces arriving and speeding up the event.
There was another version of Vormir that involved Thanos’ troops showing up and then Black Widow having to run the gauntlet and getting shot as she runs the gauntlet to leap off the cliff. And again, it just felt like it was distracting from what was most powerful about the scene, which is two friends, one of them having to die, and then literally fighting each other to be the one who dies.
While this alternate version of the scene certainly would've beefed up the action on Vormir, that wasn't needed. It was better for the characters to work this out among themselves, rather than having to see Black Widow run away from Thanos' troops to secure the stone. What may be even more important here, though, is that the end result was the same, which further back ups the belief by writers Christopher Markus and Stephen McFeely that Black Widow needed to be the one to go here.
Of course, this entire scene has actually become one of the more controversial ones from Avengers: Endgame. Many have taken issue with the execution of Black Widow's death and Guardians of the Galaxy Vol. 3.
Source: Happy Sad Confused Podcast
