It’s The Meteorite That Hit Wakanda
In the run-up to Black Panther, Marvel fans were pretty darn confident that the Soul Stone was going to turn up in the film, seeing as it was the final MCU entry before Infinity War arrived. As it was, it was a lot more standalone than we were expecting and there was nary a mention of the last Infinity Stone at all. But maybe it was actually hidden in plain sight all along?
In the beginning of the movie, we learn that Wakanda was imbued with its copious supply of vibranium and unique flora when a meteorite plowed into the Earth billions of years ago. Obviously, the big obstacle here is that the meteorite was depicted as purple in the flashback. However, it’s worth remembering that the yellow Mind Stone appeared blue before it was removed from Loki’s scepter.
Also, bear in mind that orange sand every Black Panther is submerged in that allows them to “visit the ancestral plane” and meet with dead loved ones. It has the Soul Stone’s colour and that sounds a lot like a power that Infinity Stone might bestow.
Likelihood: This one would neatly explain why the Soul Stone’s location is such a mystery. Even Thanos wouldn’t be able to locate it if it was buried under an Earth nation millennia ago. And there’s got to be a reason the final act of Infinity War is set in Wakanda, right? Sure, it could be Marvel simply knowing that Black Panther was going to be a hit so they made sure to give the hero a major role, but what if there’s more to it?