I would have realistic weapons for Marines and Covenant stay the same. I would also make some sort of Zombie Survival mode where you fight against the new flood (ie. more like zombies in the "of the dead" series) For vehicles there would be improvised ones and predeveloped ones, such as improvised a regular car with barbed wire on side and a hole cut in roof for shooting, and predeveloped would be regular military vehicles
If it were up to me, Halo would've been a free roaming game. I mean it'd be pretty awesome to be able to explore a Halo ring freely, sure it might get old but the idea of it is neat.