Bring back Dual Wielding, but don't make it so you need to use both weapons at once to get a kill.
Duals should have both advantages (like more firepower) but also disadvantages, such as decreased accuracy, OR the inability to scope in. If Halo 4 has weapon attachments (The magnum has a grenade launcher thingy in the trailer) they should only be usable with one weapon at a time.
The perfect contender for this would be Dual Magnums. Using one Magnum at a time would allow you to both zoom in, and use the grenade launcher thingy, but duals would give you... well... two pistols to use, and make you feel like a Bad@$$ (lol). Dual grenade launcher pistols sounds a bit overpowered/ pointless to me. Which would be much better to use at close range.
A better turret... because the turret in Reach is terrible. The turret in Halo 3 was good though. <----MAKE IT MORE LIKE THAT.
Make... a Needler Turret!!! It would combine my two favorite things, needlers and turrets. Everything else doesn't matter.