Looking at Halo 3, I see a bunch of things that make the game more fun to screw around and customize the game for a great experience. For example, forge and theatre. I think that if Bungie took the time they could make a straight up fun experience by making things silly and creative.
I'm making a list of these things, and I'll add to the list whenever I can, but you need to PM me, I'm not going to read the threads.
Ex) My idea is to make weapons customizable, both physical and damage-wise. Think about making your BR look like something cool, like a duck or a chicken. Or a rocket launcher in the shape of a sandwich, or grenades as a frog.
LIST-
1. Customizable appearance of weapons.