Some design concepts that would be nice in Halo 4. Keep them simple, and make them clear enough for everybody to understand.
My suggestions:
1: Enemies not automatically knowing you're there/ line of sight.The enemies in Reach always en mass know you're position. This isn't logical since not all enemies on a battlefield would know unless reported to, or they see you themselves.
2: Rechargeable energy weapons The covenant should know how to reload their own weaponry, be it via ammo or recharge batteries on the battlefield.
3: AI characters/ missions in forge. Letting players make their own missions and upload them along with more dynamic maps would greatly extend the multiplayer aspect. Also, grunt bowling would be awesome.
4: Commands. It seems odd that the missions in Campaign are often skippable, or seemingly bossy "fetch" style. Let us choose some paths ourselves, or tell marines to do something once & a while.
5: Single player DLC. Some add-ons or new game + concepts would be nice. Perhaps skulls that activate specific armor traits or abilities, or a Firefight skull that routinely brings in an airship to add to the difficulty during campaign missions. Maybe make these skulls separate from the credit system, just as turning on cheats in GTA disables achievements.
6: Actually usable armor abilities. Like most, I stuck with sprint as my armor ability. Sad thing is, my Spartan gets winded faster than I do. Also, a useable invisibility would be nice. Think about it.
7: Firefight Versus. One player plays as a Spartan, one as a low class covenant. As the rounds/ deaths advance, the covenant player "ranks up" going from grunt, to jackal, to elite, etc.
8: More Multiplayer Game Options. It would be nice to have armor ability that was double length instead of infinite, or have an instant respawn, or have melee damage at max while limiting bullet damage. Currently, these options aren't available in Reach, make them appear in Halo 4.
9: Less Invisible Walls. When I want to explore your beautiful game, please do not give me a kill countdown or knock me off a ledge. Halo CE let you go anywhere, and if Skyrim can do that sort of stuff, I'm sure Halo 4 can.