Here are some of my suggestions
1. I believe forge mode should play a very big part in Halo 4, it should have its own playlist that contains a large variety of user created maps.
2. I'm not too sure but I was told there is no official fire fight mode in Halo 4, so why not let players create fire fight maps with a large amount of control over the map. Like the paths in which enemy ships follow before dropping off enemies, round lengths, enemy types, full customization of the fire fight map, etc.
3. Night Time, by night time I mean REAL night time, not just a colour filter. At night lights can behave differently, the maps backdrop can change and maybe even tiny little fire flys can roam around the map to add to the atmosphere.
4. Last of all no instant death if you go to far down in water, it would be cool if people made some maps that utilised water on the map.