So this is an idea I had. Instead of a devil Fruit luffy get's something else instead. Something every young child wants. No it's not a lightsaber lightsabers aren't messy enough. That's your only hint read and enjoy :D

Luffy grumbled and glared as he watched Shanks get thrown around by the bandits. Shanks was tougher so why didn't he fight back? Why did he just smile and laugh, he was a disgrace to pirates.

Looking around for something to occupy his thoughts Luffy saw a small box laying on the bar. Not having noticed it before he curiously grabbed it and opened it staring in excitement at what was in side.
Only for his face to fall. inside the box was nothing except a boring, rusty old chain. "Why is this even in here?" Luffy muttered to himself picking it up.

"LUFFY DON'T" Shanks yelled just a little too late as Luffy fell to the floor unconscious.

...

Opening his eyes Luffy found himself inside a rusty old workshop, tools and various half made mechanical devices scattered all over the place. Sitting casually on the floor was a beautiful woman wearing clothes that seemed to be made out of rusty chains.

"So a new wielder has arrived for me" the woman said smiling softly. "It has been a long time since a D. has held me and a longer time since a D. had the right spirit to wield me. Such a shame it is one so small but that will change with time."

"Er who are you?" Luffy asked confused. "Where am I?"
"My name is Chain little one and as to where you are you are in my spirit space."
"So it's a mystery place then" Luffy said.
"Haha yes it is Little One" Chain laughed. "But as to why you are here you see I am one of the few sentient weapons still in existence. Our creator a man named Daedalus made us to serve those who bare the name D. like you. And when you picked up my chain I decided to claim you as my wielder. Only you can wield me and we will never be separated until the day you die."
Luffy frowned deep in thought until he asked a simple question "so I am supposed to use you as a weapon?" he asked images of picking the woman up by her foot and swinging her at people running through her mind.

"haha yes Little One but you will find that my true form is a little different from what you are thinking about. When you wish to use me in a fight then i will appear in your hand as a Chainsword."

Luffy stared.
He stared some more.
"AWESOME" Luffy yelled stars in his eyes.
"Thank you Little One" the woman smiled "I am glad you think I am awesome."

"What else can you do?" Luffy asked excitedly "can you shoot lasers? Can you fly? Do you poop?"

The woman gave a mysterious smile "you will have to find that out in your own time for now I believe your friends are worried about you."
As she said that everything went black once more.

...

Luffy opened his eyes to Shanks shaking him violently "Luffy you idiot do you know what you have just done you could have died" Shanks yelled.

Luffy ignored Shanks and looked to his right arm which had a rusty chain wrapped all around it. Without being told he knew that it was Chain and he could summon her when he desired. "Awesome" he muttered to himself.