Is hard to talk about this and getting somewhere where we can agree with.
Every MGS touches one theme and expand its story over that theme pretty much to its limits always having consideration for the fact that the entire series is about "what to pass on to the next generation and how do we live with the errors of past generations", more or less. All of this can sound really dumb to some people but the point of the series was made perfectly clear after the ending of MGS1 and the entirety of MGS2 (which was also suppose to be the last game in the series).
Kojima posted on Twitter a few days ago about the themes, scenarios and enemies from past titles...
・Gene (MGS1) → Meme (MGS2) → Scene (MGS3) → Sense (MGS4) → Peace (PW)
・"What we inherit to next generation". MGS1 was Gene, MGS2 was Meme (cultural gene), MGS3 was Scene (era), MGS4 was Sense (intention), and PW was Peace.
・"Controlling gene" in MGS1, "controlling information in digital society" in MGS2, "cold war structure in 60's" in MGS3, "controlling battlefield in near future" in MGS4, "nuclear deterrence in 70's" in PW.
・Enemies in MGS: We appointed special force in active service (elite) in MGS1, American comic type of superhuman in MGS2, legendary special force (mysterious men) in MGS3, monstrous victims of war (monster) in MGS4, and unmanned A.I. weapons (huge mech) in PW.
With that, all the supernatural stuff that happens in each game is an expansion of the story that every game in the series is trying to present but most people don't get that because lol video games.
Now, how do you put the theme of the series in a movie in a way that can make sense to "normal" people? I have no idea. Because if the intention of the movie is showing what the games are about to people who has bever played the games they are gonna end up doing a movie with the setting of MGS2, which is the climax of story itself in the main series (methinks, anyway). Though, I'm pretty sure that most people isn't going to understand the damn thing unless is told to their faces... besides, there's always the problem that the movie could end up being bad or that the story itself cannot be made into a movie comprehensively because of its expansive format.
Who knows. The only thing I know though, is that MGS is big fat mess that manage to make sense somehow and I love it for it. (*Â´∀`*)