I think that a really cool thing for 343 to do would be to make a World Editor. Not just normal forge, where you can spawn and place items, effects, etc., but make completely new environments, related or unrelated to Halo lore.
Instead of being just a game component, it could be a completely separate disc, allowing for more, better features, such as elevation tools, terrain painting, mountain creators, water spawning, and all of the old stuff like vehicle and weapon placement.
Also, there could be body/gore placers to give infection maps a creepier feel to it, and also for Machinimas. AI placers, for custom games and for fun, could also be implemented, so when you are playing alone, you aren't really playing alone.
Placing larger vehicles such as the Elephant, Pelican, Phantom, and that new, big thing seen in the E3 demo, would be awesome. They don't necessarily need to be usable, they just need to sit there and look cool, but having those as drivable vehicles would be spectacular. And possibly even attach smaller vehicles to the bottom, like a warthog to the bottom of a pelican or two ghosts to the bottom of a phantom/spirit.
Anyways, tell me what you guys think! ^__^