All I hope is that Meryl does not get killed for good in the next game. Because if she appeared, it could happen.
Snake goes then apeshit, and turns evil. Then Raiden has to come and kill him to save the world. End of MGS.
I'd love to see Raiden either playable or in the cast, although I would prefer him playable. Absolutely not replacing Snake the way he did on MGS2, but definitely to be able to play as him for a while (maybe in the inverted way it happened in MGS 2, play a little bit with him on the beginning, then play the rest of the game as Snake, that would be badass)