He's most likely new. There doesn't seem to be anyone who can qualify from previous games to be him, though I do find the Raikov theory to be quite amusing and something Kojima would do. Gene is another interesting possibility but it doesn't fit with his final words to Big Boss and I'm still waiting for Kojima to finally confirm that Portable OPS isn't canon anymore. One of the other interesting theories is that it may be Sokolov since he used the codename Ghost before but again it's not likely.

I'm pretty sure he'll turn out to be someone new and we'll find out about how Snkae and Zero screwed him over before in some event taking place before the Virtuous Mission.