I'm not super caught up with MGS series, but where can they go with snake as the main character? Is he still an old man after the end of MGS3? I wouldn't want another game with old man snake as main character to be honest, unless they can bring back young snake for the next game.
I believe you mean MGS4 with him as a old man. He dies after the ending but it's not witnessed. MGS4 ends the series, Metal Gear Rising is a cluster ____ true fans won't acknowledge.
The way to have more stories with Snake is to remake the first to MSX games like it was previously stated. Kojima had stated that MGS5 originally was going to be The Boss as the main character during World War II but was scrapped. It would not surprise me that coming to fruition at some point.