Comes out of retirement in the next MGS game (after MGSV)?
On one hand you'd have an extremely retarded decision that spits in the face of MGS lore, and on the other you could have one of the best character returns of all time.
The explanation?
NANOMACHINES SON
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No, I would prefer some kind of prequel rather than having Old Snake still. The idea of keeping making games with Big Boss is more preferable with me, he's entirely Human and they can take more personality and character development with Big Boss rather than with Solid Snake. (Who's a cold-hearted killer and only shows some tiny bit of emotion during MGS4 and that's still a lot from him)