Heimdall Has It
Sticking with the Thor franchise for a moment, one of the oldest theories surrounding the Soul Stone is that Heimdall has it in his possession. Idris Elba’s Asgardian has always been depicted as a supremely wise being – perhaps tellingly, he can apparently see all souls throughout the cosmos. And then there’s the distinctive orange colour of his eyes and his golden armor.
This is the big one that supports the T.H.A.N.O.S. theory. Could the H stand for Heimdall? The character has been around since Phase One, so it’s possible the folks at Marvel Studios have been playing a very long game here.
Likelihood: We’re pretty darn sure Heimdall is in Infinity War, so there’s that. However, wouldn’t he be on the Asgardian refugee ship which we know Thanos attacks at the beginning of the film? It looks like only the God of Thunder survives this encounter, so wouldn’t that mean Thanos has the Soul Stone from then on, which he definitely doesn’t? Still, this is one of the more convincing theories around.