Another Reddit Theory:
"So I have a theory: The leakers are most likely right, but they are leaving out one final twist in the plot: the fact that there isn't one. The beginning of the game (the Hospital) does two things and it goes two ways. The first is that it tries to convince you that this Venom Snake has already undergone plastic surgery and you now look like Big Boss, whilst giving you the option to create an avatar (for mostly online purposes, but to also try and convince you that this in fact a used body double (supposedly the medic from GZ)). Just before the doctor is killed he says that "tomorrow you will become a phantom". This can be interpreted in 2 ways: Either you already changed into Big Boss but will be unleashed on the world tomorrow, or that you ACTUALLY HAVEN'T RECEIVED THE SURGERY YET and will become a phantom of Big Boss (nobody can know you are truly him if your face has been altered)
Out of all of the things why is Ishmael wearing bandages and not just a mask or something else to cover up is identity as Big Boss? Why is he in hospital clothes? That is because Ishmael went through plastic surgery himself The game wants you to think that (eventually) you are a nobody and Big Boss is Ishmael, but in fact, Venom Snake is still Big Boss (the original!). Right until the end you don't realize that someone loyal to Big Boss (possibly still the medic) gets plastic surgery to LOOK LIKE BIG BOSS! So in reality, Ishmael already completed the surgery to look like Big Boss, but Big Boss couldn't go through the surgery to look like Ishmael due to the Doctor being killed!
This explains why Ishmael acts, talks, and thinks like Snake but cannot fight like him (he actually jumps on Quiet? instead of performing CQC?) and explains why Punished "Venom" Snake can still be "the greatest soldier in history" and perform every single ridiculous feat after coming straight out of a coma with no proper training.
I also think that BB's coma had nothing to do with his memory loss if that's true. I believe that they did something to the man while he was under (tainted/destroyed his memories, somehow caused the shrapnel to grow out of his head during the coma)
Also, HOW IS THAT A MAN THAT RECEIVES PLASTIC SURGERY STILL HAS ALL OF THOSE FACIAL SCARS?? It makes no sense. If they medic's face to BB's to that accurace then wouldn't they also be also able to remove the scars as well?
Ishmael on the other hand looks like a perfect, undamaged, true motherfuckin Big Boss. By the end of the game , OF COURSE he's going to still help out Big Boss and build Outer Heaven. It makes way more sense than a maniacal impostor still believing he's big boss run rampant around the world and the real Big Boss letting that happen.
Tl;dr: You are still BB as Venom Snake. The switch failed because of the ambush at the hospital. Ishmael was supposed to take your place and already has your face, but the plan ultimately failed because the Doctor could not perform the surgery on you in time. Zero was looking to save your life from Skull Face, to use a body double so that you may always live under a new face."
What do y'all think?