Enemy Diversity in the Campaign!
Okay, so in the Halo 4 campaign there were two clear categories of enemies: you had the covenant and the forerunner/promethean which was awesome! But how cool was it in Halo Reach when the Guta monster in Nightfall showed up... Although the standard enemies are fun and have great variety from each other, it would be good to see some unique enemies or even savage creatures that differ totally from the "norm" of the Halo campaigns. Having different sets of indigenous beasts depending on the setting, or sets of covenant outfitted with gear that alters the ideal method of fighting significantly, or maybe even some entirely new species to battle against over the course of the campaign. If new and interesting types of enemies were introduced along each mission of the campaign I think it would become exponentially more intriguing and keep players dying to know what they'll encounter next!