I think we need a realy big new set (for alien weapons - sure; but also human weapons) for Halo 4.
I wrote down my Top-10 weapon ideas, that could be used for Halo 4, 5, 6 or a spinoff… if 343i don’t has own great ideas.
Here is my Top 10:
(1st = coolest imo)
I got often inspired by other games. That means that a Halo 4 weapon needs to look complete different. But the basics how it work should be similar. I wrote my source in “” to prevent copyright troubles.
1: A GDMLS (Guided Drone-Missile Launch System):
Looks like an fat missile launcher with only one missile in it (reload should take a bit longer than to reload the spanker). It fires a big missile.
The thing is: You can control the missile in FP-Sight. It is steared like an air vehicle. It has an emergency button, so you can leave the missile sight, if someone hits your guy.
You should be able to fly it through the entire map (campaign or multiplayer).
You also can drop cluster bombs or fire smaller child missiles. So it is a mix of a cruise missile and a combat (“Predator”) drone.
The main warhead could be some mini nuclear warhead with a devestating radius (and grafic effect). Keep the weapon very rare and the reloadtime long, so it would fit.
It would be a great addition to everything halo has: Cuz it is like an RC-Drone mixed with a small nuclear missile.
btw: it should not detonate if you hit a wall. It should have some collision tolerace. Also variable speed would be great for it.
If it is an UNSC or human weapon it should be called: “devils shark missile”… or some other cool name. This one should fit into hightech-human-weapons…
[Idea by myself]
2: A dartgun that fires small arrows very fast: I ve created a weapon for a private gamemod; and it was called - SR-119 Sundew -.:
The gun looks like a big sniping rifle with a thick barrel that ends on it’s downside first. Also the second part of the barrel is not massive… it consists of small pipes.
The gun works like an car engine: there is a piston in the gun that drives the dart. The weapon works with gas or petrol or something like that. So it has an small exhaust on it’s side. Every time you shot there comes a small flame out of the exhaust. It sounds like a blowgun “Plong”.
The dart is very fast and you cant see it normaly. But it creates some “O”-SMoke clouds.
The darts are very small and dont push enemys. They only stick into them.
The dart has a lot of flukes and so on, which means a real enemy would die very slow with a lot of blood loss and pain. 
this could mean: if you have no shields or the dart penetrates your shielding and hits you; you will keep losing health very fast. So you can run like 20 or 30 steps before you die. You can only survive if you find a healthpack. (Similar to the halo ce(a) flood spore that hits you during your shields are down).
Would be also realy cool to use this weapon in campaign vs hated enemys :p.
The Weapon has a magazine of 10, 25, 35 or 50 darts. You need to reload it only if the magazine is empty. It is full auto with a slow fire rate (Autosniper, like the Sg550 in CS). But you have to watch your gaslevel aswell. If this one is empty you need to reaload that thing. Gas is enaugh for around 100 - 200 darts.
It is more like a weapon with 2 different magazines. you have to watch both. If you press reload you reload the more empty magazine first. Or if it uses both dual weapon slots, you could reload by choosing the reload-buttons… but that would mean you can’t use grenades as long as you use this weapon.
[idea by myself… it is an older idea… but my “private copyright” is outdated now 
thats why i typed the idea in here… now it’s free… 343i; grap it before some otherone grabs it! =D]
Or you saw a mix of an engine and a dart-sniping gun somewhere?!
3: Some kind of “rocket spear” gun:
It drags an enemy with it and Nails it on the wall. Other enemys get ignited if the rocket spear hits them with it’s flame. Then it explodes.
It could be used copressed air as start-propellant. The rocket boost starts after some seconds… .or you would hit yourself with the flame.
[Idea by “Bioshock 2”; the compressed air thing is from F.E.A.R.2 (Hammerhead)… the guys from Bioshock didn’t noticed the fact that you would hit yourself :p]
4: An Infernal Shotgun:
It fires shot that ignite enemys. May be the shot should stick a few milliseconds to the enemys before it ignites in. This milliseconds before it bangs are alwas awesome.
[Idea by “Bioshock 2” - Phosphor-Shotgun-Shells]
5: An Aerosol-Grenade-(Launcher):
It fires a canister that spreads gas around itself. Then, when the gascloude is big enaugh, it ignites the gas. A shockwave apears and kills the enemys (and let em fly very far). This could be also used with sticky grenades… would be realy funny.
[Idea by myself]
6: Gravity Hand-Grenade:
A thrown grenade that don’t creates any damage by itself. It creates a gravity blast that pushes the enemys fast away of it - very very fast and high (depends on angle). If an enemy hits a wall it can die.
This weapon could only hit persons, so it would be ideal to kick persons out of vehicles.
[Idea by myself, but i saw other similar ideas in this forum. Especially the vehicle thing]
7: Some kind of Crossbow:
It should be take the slot of a silenced sniper rifle. Also it the bullet has no trail. The reload takes long and you need to reload it after each or each second shot.
[…there are a lot of crossbows out there…]…so this one should be special in its appearance.
[may be it should be a bit like the bow from “Turok”]
8: A reflecting beamgun:
Like the sentinel gun, or the focus rifle from reach. But: if you hit a wall it refects from it. So it is like a bouncing projectile gun with super high speed. It would be great to fire around corners. But it needs a limited range… and may be you should be immune vs your own beam.
[idea by myself]
9: A Grunt Gun:
It looks like a grenade launcher. The grenade is full of nanobots that create a grunt that fights for you. I dont know which alien has build this weapon and why. But may be the grunts have their beginnig as a grenade payload. So that would explain why they spawn so fast.
Well this is more a joke than a real idea. But I like it. (would bring a bit more fun into the halo universe again … after halo reach)
If the grenade spawns the grunt it could make the HEEEYYYY sound from the grunt party skull… ah or not.
But it would be a gread decoy.
[idea by myself]
10: around kill weapon:
An Armor Ability or a Grenade or whatever that creates a deadly and hurting something around you. If you stand to close to the using guy… you are dead. Not so close… only damage… far away… nothing happens. There are billions of ways how it could look like, so i only called it around kill weapon. And sure: you should be immune vs your own blastwave.
[idea by everyone?]
