It’s Hidden In The Past
Let’s think outside of the box for a minute. Maybe we shouldn’t be considering “where” the Infinity Stone is but “when.” The Infinity War prelude comic confirmed that the Soul Stone hasn’t been seen for a long, long time anywhere in the universe. So perhaps Thanos will need to travel back to the past in order to retrieve it from some point in history? He’ll have a way to do that, too, if he gets his hands on the Time Stone. And we know that Thanos sends the terrifying Ebony Maw to torture Doctor Strange into handing over that Stone in Infinity War.
We don’t have any confirmation that Strange gives in and the Mad Titan does get his purple hands on the Time Stone, though. Perhaps Earth’s Mightiest Heroes themselves use the Time Stone to travel through history and find the Soul Stone? Set photos for Avengers 4 have pretty much confirmed that the film will revisit the events of several past MCU movies, most notably The Avengers.
Likelihood: Avengers 4 being structured around our heroes reliving the major moments of the past ten years of the franchise in order to find the final Infinity Stone would be a very neat and fitting storyline, so we’re leaning towards there being some truth to this theory.