Grunt....Es., Grunt. Espionage Grunt....it would be funny, lol...
Halo played from a grunts perspective, but you just encounter marines, no MC... because getting AI into MC would SUCK. You would fight in a big battle in the level Assault on the Control Room... your in the ice, a couple of shade turrets.... you can't use any human vehicles or weapons, only shades turrets. But you're invisible, you can wack in the back 
you fight against like 25 marines and you have 5 blue elites on your side along with 25 grunts, since grunts are so weak. It would be a challenge, but it would be fun. Probably would take forever and isn't original, but you being a grunt assassin working for ever pays the most covie credits wins, so you can have a mission to wack some gold elite in the back or some mission (like the last level...) to have to go through all the obstacles to kill the mother of the flood, which would be a tottaly oversized infection form that can kill anything in one hit. But hey, your invisible... even if there are 20 combat forms around it, sneak past them and shoot and shoot and shoot that mother infection form to death and the end movie would be the ship blowing up , and you fly away in a covie dropship instead of a Longsword with the MC...Probably take forever to do all that if even possible, it would still be cool and I just wanted to type something lol....
