Ah, here are some weapon ideas of mine for Nero:
Whip
This could be made out of a very tight coil or wire. It could have just 1 head, or maybe it could be upgraded to have 2, then 3, to do more damage. It would be used for sweeping circular attacks mainly, and could have the element of wind or like a sonic effect. It would make an awesome SMACK noise when you hit enemies with it. It could have some cool spin attacks, and mid air attacks, as well as more straight forward back-and-forth swipe attacks
Axe
Proper battle axe. It could have power of darkness in it, leaving a trailer of shadows / dark clouds. Could be fairly slow, and attacks could be charged up. From mid air it would have similar attack to the Rebellion (come straight down out of the air on top of the enemy). Special attacks could make large explosions of black clouds.
Ice Lance
A long weapon, with good range, with ice element - made of ice. Could have a move similar to stinger. It could be used for some cool acrobatic attacks, like using it to do a vault kick etc. The lance wouldn't be good for crowd control, as it would mainly use lunge attacks, not circular. Special attacks could have spikes of ice shoot out of the tip of the lance, like a short-range projectile.
Ninja Sword
This would be a fairly short sword, and would be quick to use. It would look similar to Vergil's Yamato, but it would have a darker blade and obviously completely different moves. He could maybe get the power to teleport with this sword too, have a move like the air trick.