well. seeing as how its a RPG i think you would play the role of a Squad leader of something. you start of in the beginning of the War and you play as a Marine Squad Leader or something. then as the Game progresses you play as a Covenant or something. Then maybe you play the Squad Captain of A Spartan group. i doubt sgt Johnson wil have a big role.
but the master probably will. seeing as how this made him sooo famous. same with cortana. and we all know that the covenant will lose something big and the Arbiter will get blamed which will lead to why he was in jail in Halo 2 or soemthing...
(this is all just me quesing)