With the February 2018 release of Black Panther, there are no more films in the MCU before the May 4, 2018 release of Avengers: Infinity War. While there are a number of burning questions left to be answered in the MCU, one of the biggest involves the very objects that will give the next Avengers film its name - the Infinity Stones and, specifically, the location of the Soul Stone in the MCU. All of the other Infinity Stones - Space, Mind, Reality, Power, and Time - have already been located, leaving the question of where the Soul Stone is all the more conspicuous.
Previously, fans had speculated the Soul Stone would show up in films as diverse as Guardians of the Galaxy Vol. 2, Thor: Ragnarok, and Black Panther. However, with those films now come and gone, the mystery remains, and fans are sure to spend the months leading up to Infinity War trying to accurately predict the Stone’s ultimate hiding spot.
One of the earliest theories about the location of the Soul Stone involved Heimdall and his realm-observing abilities, but that didn’t end up being a part of the plot in Thor: Ragnarok. This has led to many fans to give up on the theory, but Redditor/u/eatraylove still thinks the idea is plausible, and it will be revealed in Infinity War. The evidence for the theory is compelling and includes the fact Heimdall is described as being able to see every “soul” in the Nine Realms, and the color of his eyes matches the Stone. Thor’s premonition of a blind Heimdall in Age of Ultron could foreshadow a Heimdall who has had the Stone stripped from him.
Many fans assume the location of the last Infinity Stone will be revealed in Infinity War, but that isn’t necessarily the case. Redditor/u/noslowsongs theorizes the third Avengers film will center around the fight for the first five Stones, while the fourth film will focus on the race to obtain the sixth one. This would leave the location of the Soul Stone a mystery until 2019, which may be more than most fans can take.
Traditionally, the Soul Stone is connected to a character named Adam Warlock in the comics, and Warlock was teased in one of the many post-credits scenes in Guardians of the Galaxy Vol. 2. That has led to some speculation that the Soul Stone is already powering the “cocoon” housing Warlock’s body of apparent perfection, and the Sovereign have actually been in possession of the Stone this entire time. Redditor/u/JoeMothaa specifically theorizes the Soul Stone is what allows the Sovereign to maintain their “perfect” populace and what powers their Anulax Batteries, and Infinity War will feature Thanos absolutely destroying the Sovereign in his quest to complete the Gauntlet.
Of all the many theories about the location of the Soul Stone, the most plausible may be one of the most boring. Several fans, including writer Adam Holmes on CinemaBlend, have speculated the last Infinity Stone isn’t hiding anywhere - it’s already in the possession of Thanos. Already owning a Stone would explain why Thanos was so carefree in lending Loki the Mind Stone, and it would establish him as a very credible threat right off the bat. The fact Thanos holds the Soul Stone could even be revealed late in the game as a classic “gotcha” moment.