Leading the Avengers to unsnap the snap committed by Thanos was not Black Widow's original jam. Directors Joe and Anthony Russo, in a spoiler-filled interview with Slate's The Gist podcast, revealed that Black Widow's (played by Scarlett Johansson) back story for Endgame was completely different.
As per the directors, Black Widow aka Natasha Romanoff was supposed to dedicate her time to educate children and not take over Nick Fury's job of assembling the Avengers. Speaking on the same, Anthony Russo said, "One thing that we talked about a lot – and I thought was really profound, but it was almost too large of an idea for us to wrangle, but we did try for a while – the idea that one-quarter of all children have no parents."
To this, Joe Russo added, "Assuming you started with two parents. So that's a lot of global orphans. Just the staggering number of that. I believe at one point really early in development, Black Widow was actually leading the organization in D.C. that was in charge of orphans, basically. That was what she was heading up five years later. But yes, it's fascinating when you start running it down." From a script where she only dealt with kids to one where she took on the role of a coordinator to bring together the Avengers, Black Widow surely got an upgrade!
One of the main reasons why Black Widow's sacrifice was so impacting apart from Iron Man's was because she dedicated her years to the makeshift family formerly brought together by Nick Fury. She basically became the person heavily invested in coordinating with the Avengers' heroic activities on Earth and space.
Avengers: Endgame has been working phenomenally at the box office. In fact, it is inching closer to dethrone James Cameron's Avatar and become the highest grossing film of all time. Considering how the film has a long run at the box office as the shows are being actively run across theatres in major countries, perhaps they might be able to achieve this feat!
While Avengers: Endgame is considered to be the final film for the franchise, the Russo brothers have stated that Spider-Man: Far From Home starring Tom Holland and Jake Gyllenhaal will be the final film to the third phase of Marvel Cinematic Universe.