According to series creator Hideo Kojima, along with being open-world, Metal Gear Solid 5: The Phantom Pain will allow user-created missions.
"The first reason why I wanted to go open-world is because I wanted to create a world that had a lot of freedom and allowed users to create their own gameplay," Kojima told CVG. "The other reason for the game going open-world is that we wanted to use multiple devices for this game. So using tablets and smart phones you can also enjoy MGS5 and create user-generated missions within the world. That's another reason we had to go open-world--if it was linear we wouldn't be able to do that."
Metal Gear Solid 5: The Phantom Pain has been confirmed for the PS4 and Xbox One at this years E3 with a possible PC release in the future.