system
Given the following post, respond as if you're commenting on Reddit:
human
[Marvel] Unable to find any suitable candidates for Project Rebirth, Peggy Carter volunteers to take the Super Soldier Serum. How do events change?

Take your pick between MCU or 616.
gpt
Feeling that the world isn't ready for a female soldier on the front lines, her superiors require her to wear baggy fatigues and a full mask while on missions.  She is the "Unknown Soldier" and manages to save the world from the Nazis. 

In her final battle with the Red Skull, newsreel footage captures both of them as their blows destroy each other's masks.  Carter is revealed as a woman.  The world is shocked, but the PR folks for the Allies decide that it's smarter to glorify Carter rather than act ashamed of her gender.  New ad campaigns are launched that prey upon masculine insecurity in order to get men to enlist: "She's man enough to fight.  Why aren't you?"

Some politicians want to bench Carter for the cleanup in the Pacific theater but she, of course, refuses and fights anyway.  Allied media follows her every move because the public is polarized over her.  Some tune in because they want to see/hear a hero in action; others tune in because they are scandalized about a woman wearing pants and throwing punches.  Carter becomes one of the most well-known people in the world, and a new generation of girls grow up wanting to be her.  All of this serves to push Western countries into conversations that they wouldn't have for decades about women serving on the front lines and what gender equality actually means.  Enough people dismiss Carter as a unique fluke that sweeping change isn't immediate, but the timeline for women's rights is still accelerated by decades in Western democracies.

Carter lives a long life and has enormous influence in the post-war intelligence community owing to her capability and powerful connections.  She is too well-known and too politically loaded to initially become director of SHIELD, but over the years she falls out of the public eye, politicians die or forget what they had against her, and she eventually obtains the position.  In that capacity she serves as a through line for 20th century international security and technological development.  She and Fury accomplish much more together than either could on their own, rooting out HYDRA and putting the world on a very strong footing as it entered the 21st century.