I read on a site that it might show in Thor:Ragnarok and be wielded by Hela (Their Lord of the underworld, basically.)
Maybe it will appear in Guardians of the Galaxy 2 instead? I'm betting it's in one of the two. My guess is the first that is theatrically released next year so the set up for them all being stolen can be made in the second movie to be released next year.
I think it's unlikely they'll use soul stones as the main plot point in both Guardians movies, seems a bit redundant.
If I were Marvel, I'd bring it into play in the Black Panther movie. It's set before Infinity War, and they can use it as a device to bring Black Panther out of Wakanda and into the Avengers since a few of those guys are set to end their contracts soon (Chris Evans, RDJ, Chris Hemsworth, etc.)
Black Panther is set to play a sizeable role in future Avengers movies/teams, so I think it makes sense, but I get why the Ragnarok/Soul Stone connection makes sense as well.
