I dont think so.
-I'm pretty sure Kojima would've wanted to remake the two old MGS games that came out on NES.
MGS5 kinda leads to that, as it shows some of the infants terribles, one of which was Solid Snake.
And in the first MGS game, young Solid Snake goes against the real Big Boss (also featured, albeit briefly, in MGS5)
So, the next MGS game would've been sequel of sorts to MGS5, and a remake of the very first MGS on NES.
After Kojima had finished the two remakes, then the story wouldve been complete, and no more material for new games.