Officially, no. Unofficially, there's speculation that he and Konami had a falling out, leading to the dissolution of Kojima Productions (though this treatment is not exclusive to Kojima or his team). He's expected to leave after his contract runs its course, which will be after the game releases. As to why it ended up like this, could be that Konami got tired of Kojima's (spending) antics; MGSV has been in development for a long time and it has been delayed, and it's also the biggest Metal Gear game running on new tech, so it probably cost them quite a bit of money over the years. Another reason is their new focus on mobile; why spend tens of millions on a console game when you can spend a fraction of that on a mobile game and still make a lot of money? They've (or at least the CEO) also lobbied fairly hard for gambling to be legalized in Japan (outside their home country they're known for games, but inside, they're a lot more than that). Or...it could be a combination of all of the above.

eh, 4 was only good because it ended the main series in dramatic and unexpected ways. like spoiler warning [spoiler]the whole thing was a setup so gay ocelot could kiss old snake after a wrestling match before dying. Meryl ending up with Johnny "always shits his pants" Sasaki in a surprise wedding big boss was alive the whole time and conscious the main villain behind the whole franchise was major zero a british cia officer on life support, etc.[/spoiler] i found the actual gameplay to be pretty lacking.

It doesn't have to be another zombie themed.