The main problem with the current Halo 4 weapons is that every race seems to have their similar version of each weapon type, like the AR, Supressor, and the Storm Rifle. There needs to be more unique weapons, like the Needler.
I have an idea for a good unique weapon. This weapon will probably be best as a Forerunner or Human weapon. Basically, it will be a new type of rocket launcher. It has 3 missiles per mag. In order to fire it, you have to lock on to targets first. They will heat seek any enemy player. You can lock onto vehicles or players by holding RT. It will take about 2 or more seconds to lock on. You fire each missile by tapping RT. You can lock onto up to 3 targets at once. When you lock onto a target, they will be labeled or highlighted. The weapon will be able to lock onto a single target multiple times, too. The lock-on indicator or label over the enemy will have a number next to them showing how many times they are locked on. For example, when using this weapon it might take 1 shot to kill a foot soldier and 2-3 shots to destroy a vehicle. I can lock onto the foot soldier once and the vehicle twice, then tap RT 3 times to launch all the missiles. I can also lock onto a soldier multiple times so the second one will hit them if the first one fails. When you are locked onto an enemy, they would get un-locked after about 20 secs or so, or if they go out of range, or if you switch weapons. If you get locked on, it might be good if a warning sign appears somewhere on the HUD.
Please 343i, feel more than free to use this idea in your game.