In this interview via CVG, Kojima confirms that they are working on mutliplayer for MGSV.
Will MGSV have an online multiplayer mode like the previous game?
Yes, we plan on creating Metal Gear Online. Multiplayer right now is being created at our LA studio as well as us. We're working closely to create a vast multiplayer for all the fans.
The online mode in Subsistence is the most fun I've had in an online game, and I probably spent 500+ hours in Metal Gear Online in MGS4. It wasn't the best third person shooter, but had enough weird stuff to keep it interesting (heroes with special abilities, SOP destab) I was excited enough for MGSV, this just adds to that. It would be amazing if they had a "Free Roam" type of gameplay like Red Dead Redemption.