With Infinity War coming in close on April 27th, 2018, there have been a lot, and I mean a lot, of speculations for which movie, if any, would conceal the soul stone in it. Anywhere from Heimdall to Black Panther.
The 'THANOS' Theory
Most people believe that the infinity stones are subtly spelling out the word "THANOS" for where they are located.
The "H" is where the soul stone is supposed to be located. This where most people believe that Heimdall is the one holding on to the soul stone because he can see souls everywhere. However, there is one problem with this idea: Heimdall's senses are what give him the ability to see all life across the nine realms. That is why he is guarding the bifrost. He can see an enemy attack days before they actually do. Also, the reason the Asgardians decided to give the Aether to The Collector is that they didn't want to keep two infinity stones together. Unless Marvel retcons Heimdall's abilities and gives him the soul stone, someone else will have it.
Let's say that the stones will spell out "THANOS." The "H" is still missing. Another common theory is that Black Panther has it. To fill in the word, people are saying that the heart-shaped herb which was grown when a meteorite hit Wakanda and gave them Vibranium—the core of the meteorite could be the soul stone. However, director Ryan Coogler said that "I love the Infinity Stones as much as any comic book fan, it’s just Wakanda already has its thing, which is Vibranium. For us, that was special enough, so to throw in something like another special thing didn’t feel right. It felt like we should stick with our one MacGuffin for the country and explore that, let that be the important thing because, frankly we didn’t need to have another piece like that ... [Marvel Studios] never really was interested in putting a stone in there, either." With this being said, the Black Panther theory is shut down.
At the end of Guardians of the Galaxy Vol. 2, we see that Adam Warlock has been introduced into the MCU. In the comics, Adam Warlock has the soul stone embedded in his forehead just like how Vision does. When Thanos kills Adam Warlock and takes the soul stone, his soul is still in the soul stone. Later on, Adam Warlock's soul emerges from the stone and turns Thanos to stone. To fit the bill even more, Warlock's nickname is "Him," which would fit the empty "H" spot. However, James Gunn has openly denied that Adam Warlock will not be in Infinity War.
My Version of the 'THANOS' Theory
I believe that the words for "THANOS" are for who is holding the stones.
T- Taneleer Tivan (The Collector- Reality Stone)
H- Head (Vision's Forehead- Mind Stone)
A- Asgard (Space Stone)
N- Nova Corps (Power Stone)
O- ??? (Soul Stone)
S- Strange (Doctor Strange- Time Stone)
I've looked through possible hosts of the soul stone and saw that Obliterator, Oblivion, Occulus, Oneg the Prober, Orikal, or the One Above All would be perfect fits. The Obliterator is an Elder of the Universe just like The Collector and The Grandmaster. Oblivion is a cosmic entity and an aspect of Death (The one Thanos loves). Oblivion is omniscient and omnipresent, so that would help if had or knew where the soul stone was. Occulus can tap into crystals surrounding a planet and use the energy to gain amazing powers. He gets one of his eyes replaced with a power-gem (Infinity Stone). This could be the soul stone and could explain why he has unstoppable powers. Oneg the Prober is a Celestial just like Eson the Seeker. Oneg could be using the stone. Orikal has mystical powers that can nullify Odin. He also has an "Infinite Eye" that can see everything happening in the world. Sounds like the soul stone. The last suspect is the One Above All. The One Above All is a Celestial just like Oneg, and he could be holding on to the soul stone just like how Eson used his in the hologram in Guardians of the Galaxy.
In conclusion, nothing is 100 percent confirmed yet. We will have to wait until April 27th to get the answer we have been waiting for. Only the directors know how the story will play out, and I think everyone should buckle up and enjoy the ride.