Vision’s Infinity Stone
Like Vision himself said in the movie, this gem is a mystery. We all know that Thanos, at some point, will have all 6 Infinity Stones, including the one embedded in Vision’s forehead. His arc in many ways is somewhat similar to that of Adam Warlock, who in the comics also has a Soul Gem in his forehead and who is a known enemy of Thanos. I’d say this theory definitely has legs.