In an interview with Japanese site Inside Games, translated by DualShockers, Kojima talked a little bit about rankings and replayability in Metal Gear Solid V: The Phantom Pain.
“In some missions there are “No Kill, No Alert” challenges, and all those challenges are connected by social [rankings] and can be played any number of times. In addition to that, even in the same mission there will be different situations, like different ways to attack and different weather conditions.”
“Everything you do will eventually be evaluated at the base. For example, if you bring back the wrong target in a hostage rescue and you should do it again to improve your result. That’s the (non-linear) replay value due to the freedom enhanced by an open world.”
It seems like this will work similar as it did in Metal Gear Solid: Peace Walker, where you were giving a ranking at the end of each mission based on different factors, such as number of kills, alerts, and more. Furthermore, replayability will be enhanced by the fact that missions can be replayed under different circumstances.