So a big narrative theme is centred on the 'forgotten' folk. TPP presents us with a soon-to-be forgotten culture and language (Codetalker and The WW2 Codetalkers), forgotten soldiers (the nutty dudes from Motherbase I), forgotten events (Skullface's involvement with MGS3), forgotten memories (Paz, Ocelot); even forgotten personalities, lives and identities (Skullface, Venom, Quiet, Paz, Volgin etc.). We're constantly confronted by the unacknowledged, the missing or the forgotten history that the legends we later know are built upon; hence, The Phantom Pain (natch!).
There is no one more forgotten in the entire series, no one who has arguably contributed more, than the player themselves. For all the talk of Kojima's ego, exemplified so perfectly by his name being insistently plastered across the screen at every opportunity, it turns out his final big statement is to thank us. We're the most 'forgotten', the ones which his own 'legend' is built upon. In that way, the twist becomes a kind of 'Here's to Us', so to speak. I find this is the point most often unfairly dismissed out of hand.
Mechanically speaking, it's no coincidence that the gameplay from the top down is built around giving the player agency. We build the base. We choose the missions. We decide on the when, where and how. The sandbox and the systemic approach allows us, the players, to generate our own stories through our actions, rather than have them dictated to us. That was the intention from the start, as shown in many pre-release interviews. I think it's interesting that we've spent the whole series in the shadow of Big Boss' legend, yet the final twist suggests that, actually, it has always been our own legend that we've been chasing. That's a neat reframing of an area of the narrative that has always remained fairly nebulous, if you ask me.
In many ways, TPP is an inversion of MGS2's handling of interactivity and player agency. In MGS2, we are taunted and mocked at every turn, set to follow a predestined path through various set-pieces to an inevitable confrontation. At the end, Raiden discards our agency and gains his own identity.
Whilst the cutscenes are set, TPP is all about our actions and gameplay decisions. We are weaving the tapestry of Big Boss' legend. At the end, it is the player discarding the Venom/Big Boss illusion and realising it has been us all along. That's cool!
It also carries on a Kojima staple in a really interesting way (bear with me here):
Since the end of MGS2's Tanker Chapter, Kojima has pointedly refused to let us play as OG Snake, even though we've figuratively begged to. We get to play his dad, a waifish whiny substitute, and a prematurely aged version, but never the 'real' thing.
Though not specifically about the 'real' Snake, TPP's twist is the first time we're directly confronted with a question about it:
If it looks, acts and is capable of all the same things, did it matter that you weren't the 'real hero' you were promised you would be?
Apparently, it did!
Further, as the deception is at the end rather than the beginning (a la MGS2), it asks us to consider our feelings during that journey when we did believe we were BB. It reframes that whole experience and essentially points to us, the player, as the common denominator in all the incarnations of Snake.