I realize this is a wierd thing to mention, but hear me out I think it could be a good idea for Halo 3:
If anyone has ever played Goldeneye (N64) they should know this: there is an AI difficulty adjuster thing you get to access when you beat the game on the hardest difficulty. On this thing you can adjust your enemies toughness, accuracy, weapons, pretty much anything. We should implement this into Halo 3, when you beat the game on Legendary you should get this as a reward. That way you could make the game funner or impossibly hard, or both!