These GF to BB theories seem incomprehensible, since even if he's in MGS5 he'll survive and not be BB. So the ideas of going to such lengths to change him only to change him back to normal sounds ridiculous. Oh shut up don't you tell me it's Metal Gear anything goes, Decoy Octopus was the only inconsequential guy in the entire series that did stuff like that, and it was because that was just kinda his thing.
I feel silly even humouring the idea.
The only fact about Decoy Octopus that is known is that he was one of the older members in Foxhound during the Shadow Moses Incident. So Kojima might give him a bigger role in MGS5 in a similar fashion as Psycho Mantis seems to be being given.