One of the joys of Marvel's What If...? comes in exploring an infinite array of possibilities in the Marvel Cinematic Universe, but some of those worlds prove so rife with possibilities it's a shame to watch them end at all. While What If...? Season 1 ended by tying all the various alternate realities together with several characters returning, some characters are too good to get their due in return team-ups. Captain Carter is the perfect example, as What If...? sets up her potential future without ever fully tapping into it.
Captain Carter is the epitome of what What If...? is all about. Cascading from Peggy Carter staying on the main floor of the super soldier experiment, in this alternate reality, Steve Rogers is unable to go through with the experiment, and Peggy steps in to fill the role. Quickly proving more effective than the mainstream Captain America, Captain Carter's rises to fame, but her story in the present day is not explored as much as it could.
When she is revisited in the series -- shortly before Uatu recruits her into the Guardians of the Multiverse, she enjoys a friendly relationship with Black Widow, as the two undergo roughly the same mission as Captain America did in Winter Soldier. Peggy's adventure in her solo episode ended with her fighting a tentacled demon in an alternate dimension that suspended her in time before her return in the present day.
These brief glimpses of her life in the present day show just how similar her journey mirrors her counterpart's in the mainstream MCU, but there is still plenty to explore in a spinoff. While the Season 1 premiere and its finale did an admirable job exploring the premise of how Peggy could fill the Captain's role, what is possibly more interesting are the other ways in which her world diverges from the main timeline, like how Captain Carter would've impacted the feminist movements in following years or how Howard Stark's new technology developed with Carter's help would've impacted Tony's creation of Iron Man.
Without Bucky becoming the Winter Soldier it would even be possible that Howard Stark was alive in the present day, and there's no telling how Peggy might react in every futuristic situation with events like Age of Ultron and Civil War ahead of her. The end of What If...? even sets up the possibility that Steve is still alive, preserved in the HYDRA Stomper armor, which works as the perfect tease for a spinoff.
There are plenty of other episodes with promising premises in What If...?'s first season, but Captain Carter feels like the concept with more story to tell. Marvel zombies, an evil hank Pym and Party Thor were all great concepts to see play out, but by the end of those episodes, they feel complete. With Captain Carter and her intriguing world, there is still plenty more left to play with.