After all of this speculation, the irony is that Thanos could simply walk into Infinity War with one of the remaining Stones in his possession already. A simple line of dialogue as to where he found it on some distant planet, and theres no need for anything more on the subject. They could also do something a little more palatable and have Thanos find it during the opening scene of Infinity War. The scene could function almost like a Bond prologue, in which Thanos barrels through enemies and unearths the Stone. It could actually be neat to see the parallelism between a scene like that and Star-Lords opening robbery in Guardians. Sure, it might not be perfect, but at least wed get to see him find it instead of just being told that he did. This would absolutely be the most boring way of moving an Infinity Stone into place but it would be an effective one. Marvel wouldnt have to burn a whole movie on setting up another Stone and could instead simply fast forward the process.