One suggestion that I think would awesome if you guys added would be the addition of the flood as playable characters in multiplayer. Specifically the addition of this functionality would allow for alot more creativity in the machinima scene and those who create captured photos. Also, the biggest thing this addition would provide is that it truly would make zombies a legitimate gametype for the casual audience.
Adding more about a potential revamp of the zombies gametype, I think it would be awesome to have ability to control different flood characters that possessed different player attributes and weapon sets (sort of like left 4 dead). For the spartans there should be incentives to stay longer, which could include the reward of unique weapons as the duration of the match goes on or as other members are converted. Spartans would be converted to flood only when the team’s pool of lives ended. The pool of lives that the spartans would receive would mean that there would need to be more flood (zombie) players initially to make the games more fair.