The only acceptable part would be if Keifer voiced Venom but then they kept Dave as Big Boss, but then the twist would be more obvious than it wasYes he had done interviews just after 2 where he already wanted to quit. I actually vividly remember reading such an interview. He also wanted to find another person to continue doing MGS games. Back then he was already tired doing it.
I think Kojima is overrated anyways. I think most of the things I like in his games (gameplay, realistic graphics, stealth gameplay) is actually done mainly by his team, and the rest I don't like (nonsensical story and characters) are Kojima's creation.
In the interview according to him his main work on MGS2 was writing the story of the game. In another interview about 3, I remember him saying that he wanted to be surprised by his team about the realistic graphics of the game, again, his main job was the script / story at the beginning and doing cutscenes while directing the game.