Update: It appears that the game hasn’t been confirmed yet after all, Kojima only stated there is a possibility for MGS5 with the Fox Engine. This was mistranslated to a confirmation.
According to an interview held with French gaming magazine IG Magazine, Kojima has confirmed there will be a Metal Gear Solid 5. The game will apparently run on the FOX engine and will once again feature much infiltration and espionage. It will also pick up on the concept introduced in MGS4, where you could convince people to do you a favor (he probably refers to the fact that you could help the rebels and exchange items with them). Kojima also mentioned he likes the idea of social interactions in Deus Ex, but he’s not saying it will necessarily be in MGS5. This probably refers to being able to choose from a dialogue tree and take sides with certain characters.
Lastly, Kojima also implied the game will feature Solid Snake as a protagonist. In response to a question asking how he feels about Metal Gear Rising: Revengeance, Kojima responded that this was never ‘Metal Gear Solid’, but rather a spin-off that tries to do something new. He also said that despite wanting to kill him in MGS4, the studio is not yet done with Solid Snake.
As fans may remember, a few months ago Kojima Productions was recruiting for new people to help develop ‘the next Metal Gear’, and this could very much be it.
The only strange thing is that the artwork features Big Boss, whereas Kojima is talking about Solid Snake in the interview. The only point in the timeline where the two were together (aside from MGS4) is in the original two games. In any case, at this point it all remains vague and uncertain. Kojima has said that he will reveal a new Fox Engine demo in July, so maybe we’ll learn more on what exactly is keeping him busy these days.