Before the game came out, I thought all the enemy bosses in the game were going to be possible illusions, causing us to never quite know what was real or not, and allowing Kojima to make the fights as fun as desired without having them to be realistic. In fact I thought the whole game would play on this theme, where even some enemies would be possible illusions. For some reason the hospital scene looked like a dream, and I thought the whole game would feel like that.
Volgin on the unicorn, the giant whale, on fire, the lightning bolt, Mantis, I thought it was a hint of it. I even thought Big Boss and SF might be the same person, some sort of Tyler Durden thing, due to the XOF unit, SF having a scar where V had a horn, etc. I thought SF might be the demon inside BB himself, trying to take over BB's own mind by fighting against his own soldeirs using his own "mirrored" PMC.
I knew it couldn't be the case, but even in TPP I thought it could still be true until the Metal Gear part where obviously he's really there. Even the car ride made sense: BB says nothing cause he's talking to himself! His soldeirs just don't dare say hey who are you talking to
I kind of wish that's what it would have been. A literal demon inside BB, a split personality taking over, never quite entirely, but enough so to remain there until SS kills BB. It would explain why BB can be such a hero one moment, and then say stuff like sending kids back on the battlefield is fine.