Another idea I came up with, although it's not a completly new weapon. My idea is that once you achieve the max rank in future Halos (Halo 5, Halo 6, etc.) you have the ability to get an energy sword as a loadout weapon. It would be slightly tweaked though, to not throw off the game's balance.
1. It would be named Energy Sword 1.0 (or something along those lines)
2. It would use up it's ammo twice as fast as the default Energy Sword
3. It would appear on your spartan's model like the katana did in Halo 3
4. Energy Sword 1.0 will be categorized/available as a secondary loadout weapon, under pistols
5. You can not pick up this weapon on the battlefield (even from the dead player wielding it)