Kojima said he wasn't doing MGS3...or MGS4. 99% chance there will be another game in the series (especially since he's said he was thinking about MGS5 the whole time he was making MGS4). If there were to be another game, there is a lot of material in the game's universe to draw from, but whether or not we play as Raiden, Snake, Big Boss, Ocelot, The Boss, or any of the other 11tybillion characters will be up to Kojima.
Oh and Raiden doesn't suck.