You can make a good story with Big Boss post-PW but pre-MG1. Is it necessary? Not really -- but this is true of any game that they could make with the current timeline. The thing about Big Boss is that he has a rich history to work with. He's been all over the world on countless missions. It doesn't have to be revelatory to the overall series for it to be an interesting story. That's the important thing.
--
"When I was a young man, I had liberty, but I did not see it. I had time, but I did not know it."