I'd think that if a Metal Gear Solid 5 were to be made, it'd be another prequel like MGS3, just more recent. Maybe a remake of the original Metal Gear games.
Or, they might branch out and make another character, and only mention Snake in the game, but have no visual of him.
I wouldn't mind them continueing the series, as long as they kept some reference to the other games, so that it's a just a continuation of the plot.
EDIT:
It's not official that it'll be made, at all. It's only said that if it is made, it's going to be by a different director, and not Kojima.