Currently, in the forge in halo reach, players have the ability to add different visual effects to the maps they are editing, like making the screen black and white, enriching the color textures, or adding in an old time grain effect.
But there has always been something missing from the forge and that is weather effects. Granted, this would be a waste of time for indoor maps, but they would be great for outdoors. It would be awesome to be able to set up a map with heavy rain, dense fog, or whiteout blizzard conditions. How cool would it be to have a sniper duel, no radar, in the middle of hurricane like conditions? Or to be waging a BTB in the middle of a blizzard or some CTF in a fog so thick you could cut it with a knife?
To further the effects players could have the choice to add in other elements such as thunder and lightning. Players should also be able to set the severity of the weather effects, from a light drizzle to torrential downpours, from flurries to whiteouts, or from a light mist to not being able to see more than a few feet in front of you.