When Konami 1st published Metal Gear Solid: Son's of Liberty (later they published Metal Gear Solid: Substance), I think they want to replace Snake role with Raiden. But they failed. What's wrong?
I think they didn't create good plot to implement that replacement. Because of that failure, Solid Snake must continue his journey until his death (may be).
Btw, here is my plot:
Solid Snake is a mentor of Raiden (or may be Snake's clone). He accompany Raiden into several missions. Because Snake is very old, now he is just accompany Raiden in the codec only (giving some advices).
I think Konami should consider my plot!