I don't think that this is able to be done without making a mod since i don't believe that plugins can create items.
Feel free to prove me wrong.
A plugin can create entirely new items, but they cannot create new textures
Element: Fire
Item: A fire sword that increases the power of firebending 3 times as much in whatever slot it is in except heatcontrol and firejet. So a fireblast 3 times as powerful.
Crafting: Diamond sword with another diamond sword and a nether star, and a netherrack in any pattern in the crafting table.
This seems silly but for some reason I kind of like it. Similar to Zuko's use of swords during his fight with the Earthbending police.
Also 3 times as powerful seems kind of drastic, it'd be nice if the power boost was configurable.
I think it should completely change the ability that it is in imo. Have the sword in Fire Blast slot, Fire Blast takes the shape of a crescent and is much faster.
Have it in Fire Shield, and it shows the animation of someone blocking with a sword, just with Fire Particles and it has the same effect as blocking with a sword.
Have it in Blaze and I think the Blaze should go farther, or in a straight line, or just give the sword Fire Aspect.
I don't know about wall of fire, but, anyways, you get the point where I'm going with this~