Three things I'd like to say.
First and most important: All aboard the hype train! I'm stoked for this.
Second: I think a game centered around Gray Fox would be, honestly, a little boring. His death was iconic to the series and bringing him back, or setting it before MGS1, wouldn't work well. Bring him back and it nullifies the feels his death created, taking an amazing death and stomping on it (see what I did there?). Set it before MGS1 and all the enemies will be too technologically inferior to pose any sort of threat, unless you make the completely human enemies for some reason take several slashes to kill. And no, there will likely be no mechanical enemies. Unmanned Gears are a rather new thing come MGR, besides the Irvings. (though I really wouldn't mind a fight against Cocoon, Pupa and Chrysalis in an MGR type fight, let alone Peace Walker or the Shagohod) No, a story centered around Gray Fox would be more suited to an MGS style game, not MGR. (Plus I actually have the gall to like Raiden. Sue me, flame me, I don't care. I like him.)
Third: I really hope it's not as short as the first game. Considering the first game was made in little over a year I'm not knocking it for that, but the first half of the game definitely feels more polished than the latter. The first five levels are great, but the last three are disappointing. Escape from Denver is like ten minutes long, Desert Showdown is just the fight with Sam and the last level, which I don't remember the name of, is just as short as Escape from Denver and the only saving grace is the fight with Armstrong and EXCELSUS. Either make the levels longer, make more levels in total (eight is just galling) or both.