Well if it's an entire video game, there are no boundaries.
First of all is the ability to choose which race you play as. However, you are limited to the races in the galaxy during the Human Covie war. So, you get to pick between Human (UNSC), Human (Scientist), Human (Rebel), Elite, Grunt (Heretic), Grunt (Covie), Hunter (Heretic), Hunter (Covie), Engineer, Brute, Jackal, Drone, Monitor, and Flood (You would start as an infection form, then slowly work up to a juggernaut, which sadly never made it into Halo. Look them up sometime if you wanna.)
It would take place during the end of Halo 2, so if it says heretic, they are Elite Sympathizers.
I think thats a good place to start off. Anyone disagree with the choice of races?