Dynamic weather would be a cool feature, especially if it affected game play. Rain, snow or wind could alter the AI's decision-making. Vehicles could handle differently; speed and visibility could be affected. It's just a tiny detail that would be nice.
Hey, how about some zero gravity space battles? A zero-G fire fight would be majestic! Picture a battle in orbit over some planet. Seraphs and Longswords spiral around you in dogfights. Capital ships exchange heavy rounds. Space is littered with debris. The level could have the slow rolling feel of a drift dive. A special unit of Covenant space troopers swarms UNSC ships, and it's up to the Chief to defend them and plant a bomb on the hull of one of the Covenant cruisers. At different points in the level you could have Chief running along the hull of a Covenant Cruiser, drifting through the wreckage of a ship as well as bits of free fall combat. Moving Master Chief in a totally zero-G environment would be tricky but well worth the learning curve to fight in space.