Honestly, if there’s one thing that I can say that I instantly disliked about Halo 4 (everything else became a gradual dislike), it was that I felt like I had barely any weapons to use. The Covenant and Promethean weapons all felt like reskins of human weapons, and a lot of weapons were removed from the game. The Light rifle and Boltshot (no matter how much I hated it) stand out to me as some of the only weapons that made me think “this weapon is pretty cool”.
I won’t sit here and pretend that I can balance the weapons I’ve thought of so I’m just gonna tell you guys my ideas and I hope some of you will do the same 
First off, more needle weapons. They are probably the weapons I find the coolest. Maybe a power weapon that doesn’t actually kill but instead drains shields when 3 needles are shot into an opponent. A support weapon basically.
UNSC Dragon- Deals a bit of damage with it’s stream of fire but the main purpose is to jam the gun an enemy is holding after a few seconds of fire. Now, your gun just stopping wouldn’t be any fun, so I think the Spartan quickly switching to his secondary weapon would be pretty sweet.
Covenant Charger- Two guns treated as one weapon. It gets picked up as two, dropped as two, but is only one. The gun on the left shoots out bolts of electricty (It would look cool, sue me) that stun opponents after a certain amount that I don’t know how to specify has been dealt out. The right shoots more powerful electricty, dealing about as much damage as the Halo 3 magnum. Since it’s a dual wielded weapon it has all the drawbacks of dualwielding but the stun effect makes up for it.
Forerunner Shard- A grenade that scatters ice shards that freeze a Spartan’s armor when embedded into it so that the new Spartan abilities cannot be used, since I’m guessing they require complex armor movement. The effect lasts about 10-15 seconds. An unshielded Spartan will die from the blast. Only Spartan abilities are affected.