David Hayter isn't voicing Big Boss because he's going to make a cameo of sorts in MGSV if not a later game as Solid Snake. Also, I thought the main theme was gene or... meme or something.
fwiw, I think you are getting the individual games mixed up with the franchise, or something?
MGS1's theme was gene, MGS2's theme was meme, MGS3 - scene, MGS4 - sense, Peace Walker - peace.
So there's nothing stopping MGS5 from having the theme of Revenge.