Captain America had perhaps the most satisfying overall character arc throughout his tenure in the MCU, leading up to a bittersweet conclusion in Avengers: Endgame where he finally gets the dance with the girl he loved. After Peggy Carter passed away in Captain America: Civil War, the collective audience had assumed that their story together had unfortunately concluded without a proper resolution. However, once again, the Russo Brothers' collective capacity for satisfying subversion was underestimated.
The final image of Steve Rogers' arc was two long-lost lovers getting the dance they always wanted. But if a new theory is to be believed, there are more layers to this scene than previously thought. According to a Reddit post from user u/jamaiconbaicon, the dance between Cap and Peggy does not take place on Earth, but rather in Soul World. This location is the pocket dimension that exists inside of the Soul Gem, the most mysterious of the six Infinity Stones. Thanos had previously visited Soul World in Infinity War after sacrificing his daughter Gamora in order to obtain the small golden rock. The sequence in Soul World is one of Infinity War's most introspective and is important in reference to Endgame's final scene in a couple of ways.
The Thanos scene in Soul World gives the audience context to some of the Soul Stone's vague rules. It seems to imply that one can be transported to Soul World when interacting with the stone on Vormir, and where its steward Red Skull resides for eternity. While Hawkeye is not shown having a Soul World experience exactly the same as Thanos' on Vormir in Endgame, there is a deleted scene in the film which shows Tony in Soul World after using the Stark Infinity Gauntlet interacting with an adult version of his daughter Morgan.
While Hawkeye's lack of a Soul World experience leaves a hole in this theory, the Stark scene gives more credence to the assumption that the Soul Stone gives its user what it most desires. For Stark, it was to see his daughter grown up. For Thanos, it was to see his daughter at her most innocent, before she had been corrupted by the evils of the galaxy. For Cap, it would be to have that one last dance. As the user pointed out in their initial post, the Red Skull states that in order to obtain the soul stone, a soul must be given in exchange. Would it not then stand to reason that returning the Soul Stone back to its rightful resting place would result in the gaining of a soul? Maybe Steve was able to bargain for a life with Peggy, on the condition that the entire experience is lived out in Soul World.
As the user explains further, this theory could actually fix a plot hole with Cap and Peggy's arc. Before this theory, many fans assumed that Cap's sudden reappearance into Peggy's life through the miracle of time travel could present a few issues. First, it could spell the end of whatever domestic relationship Peggy was in at the time. Second, it would cut out any issues presented when Cap rejoined a timeline where he already when under the ice. Any other conundrums are also quickly abated when one considers the possibility of a romantic Soul World rendezvous as the ultimate sendoff for this fated pair.
However, even if the Soul World theory is true, it also creates its own potential plot holes, although some fill themselves in on their own, including how Cap was able to get back from Soul World in order to give Falcon the shield in Endgame's final moments. Well, it's possible it was the same way Thanos and Hawkeye returned from their respective trips to Vormir, although it's still unclear how they were able to pull that off. It would seem that the stone returns the user to their original reality after they experience their true desire.
While the Infinity Saga may have come to a close, and although the MCU may distance itself from the Infinity Stones for the foreseeable future, it's still worth it to consider these theories. They give new context to already consequential sequences in these iconic films, and may even end up proving to be more fact than fiction as further MCU films are released in the coming years.
