Straight from Kojima's mouth
Are you ready for more games based in the Metal Gear universe? VG247 recently spoke to franchise father Hideo Kojima, who confirmed that more Metal Gear spin-offs may be coming. In regards to the prospect of more spin-offs, Kojima states, “I may make more spin-offs, possibly with The Boss, which is something I personally want to do. But it’s not something I will make a habit of.”
I was pleasantly surprised by Revengeance (ok well not really "surprised," since I trust Platinum), and I always thought that The Boss and her crew could easily fill a spin-off game, so I'm completely fine with this.
Keep 'em coming, Kojima. Keep 'em coming.