It will likely be MGS5: Daughter of the Patriots, take place 10-15 years after MGS4, and star Sunny who has taken up her mother's Ninja gear from MGS2. Then they can replace the traditional electrocution interrogation minigame ("Press X to not die!") with making eggs for grandpa Snake ("Press X to not burn eggs!").
Throughout the game she will also be hit on by the son of Johnny and Meryl. She will eventually give into his bumbling charms and carry his child. However their love is not to last as she finds out that he is the leader of whatever evil organization she is fighting to stop and his father, Johnny, was actually the mastermind behind all the events of MGS 1, 2, and 4. He inherited his power from his ancestors who had caused all the events that befell Big Boss in all of his games.
The game will end with a cliff hanger with Sunny delivering the baby while in enemy territory with the help of a cyborg Naomi Hunter.
However, the next game will not pick up this story thread and instead be a prequel titled MGS6: Swords of the Philosophers. It will tell the story of the secret European pre-origins of the Philosophers Legacy. It will star Johnny's distant ancestor in the middle ages, who coincidentally looks exactly like Johnny but wears an eye patch.