Some theories state that the Soul Stone was in the vibranium meteorite that crash-landed in what would become Wakanda.
It’s buried deep within the Earth and is what gives the Heart-Shaped Herb it’s power (and all the kooky visions).
Before Black Panther released and became the highest grossing superhero movie ever made, there was a solid chance that the Soul Stone would be revealed to be in Wakanda.
At this point, the chances aren’t great this will end up being the case, but it was a fun theory to think about.