Been thinking about this. Got a feeling we won't be seeing gray fox in tpp. Why? Because ocelot is in it. That's why. Though,given the franchises convoluted nature, I realize it wouldn't be beyond kojima to make characters cross paths at different points in the timeline but I doubt he'll do it here. having ocelot and fox working in the same outfit would just be odd. Unless kojima is trying to intertwine their fates in a new light now that we're at this point in the timeline. Cruz we all know a missing arm is the end result here.
Wait, how does Ocelot being in the game somehow preclude Gray Fox from appearing? Ocelot even says to him in MGS1 "You! But you died!!!" indicating that they've crossed paths before. And Ocelot and Gray Fox having worked in the same outfit actually makes sense because we know they were both Big Boss loyalists at some point.
Just because he cut Ocelot's arm off doesn't mean they can't have worked together in the past, it's even a recurring theme in MGS of former allies becoming enemies and vice versa being dictated by the "times" they live in. Then there's the fact Ocelot actually said to Solid Snake during the torture that "I fought wars in Afghanistan,
Mozambique, Eritrea, and Chad. Among the Mujahadin Guerillas, I was known and feared as 'Shalashaska'." We know Mozambique was where Big Boss reunited with Gray Fox again and we can presume from TPP that Ocelot was with him.
Also, I'd rather not even acknowledge this but it can't be helped... I could have sworn there was some business in MGS4 where it was mentioned that Ocelot & Eva actually orchestrated Gray Fox going rogue and killing Dr. Clark and his appearance on Shadow Moses? So there's that as well. I really dislike that retcon.
All that said I'm not holding my breath for his return anymore for different reasons.