There are a ton of heart-shaped herb Soul Stone fan theories floating around the internet currently in the wake of the gargantuan box office success of Black Panther. However, Avengers: Infinity War co-director Joe Russo has revealed that Thanos and his Black Order will be attacking the formerly-hidden nation in order to retrieve the Mind Stone currently housed in the Vision's forehead.
Apparently, Captain America makes the call that the best place to defend Vision from the Mad Titan is in Wakanda, thanks to their advanced technology. "It's never been conquered," quips Chadwick Boseman.
Russo further commented, "Vision is a living MacGuffin. Obviously, that raises the stakes because Vision’s life is in danger, and his life is in conflict with Thanos’ goals, so something’s got to give. Cap makes the decision to bring [Vision] here to Wakanda, Earth is making its last stand to keep the stone from Thanos. It’s the best place to make your last stand.”
This now leads to the question of where is the Soul Gem? The other, equally popular theory is that it is in the possession of Idris Elba's Heimdall. However, seeing as how the Mad Titan encounters Thor and the rest of the Asgard refugees at the end of Thor: Ragnarok, it hardly seems feasible that Heimdall would escape unharmed.