"Be careful, kid. Those are extremely valuable experimental subjects. So far, only two have successfully gained Devil Fruit abilities," Vegapunk said, feeling his head about to explode. Leo was getting more and more irritated as well.
"Don't exaggerate, old man. This sword ate the Bull Bull Fruit. Aside from its strange shape, its abilities are nothing special. And Devil Fruits of the Zoan type are extremely hard—it's impossible to damage it," Leo said, clearly not taking it seriously. They'd spent enough time together to understand each other's habits, so their conversation was casual.
"But sir, the other experimental subject is incredible. He uses firearms to channel the power of the Porcupine Fruit. He has unlimited ammo, and the bullets he fires can penetrate anything. Doctor, I'm really interested in this technology. Could you teach me?"
"Easier said than done. I can teach you, but you must first obtain a Zoan-type Devil Fruit," Vegapunk said unhappily. Soon, everyone gathered in Vegapunk's lab, staring at the Bull Demon Sword. Originally a straight sword, it now looked like a trident.
"What the hell is this thing, Vegapunk?" Judge said disdainfully as he picked up the Bull Head Sword.
"Obviously, it's a sword that consumed a Devil Fruit," Vegapunk answered seriously.
"You knew it was a sword, and you fed it a Devil Fruit? What's the point? Can this thing be replaced with another Zoan-type Devil Fruit?" Judge was clearly unhappy about Vegapunk wasting valuable materials on this experiment.
"Don't you see how powerful it is? It breaks the barriers of life—objects gain Devil Fruit powers and come to life," Vegapunk explained.
Seeing the argument escalating, Stussy interjected: "Doctor, can a Paramecia or Logia-type Devil Fruit merge with an object?"
"Not yet. This is the product of bloodline limit technology. Currently, only Zoan-type Devil Fruits can do this. Logia and Paramecia types lack soul feedback, so they cannot merge with objects."
Stussy's question echoed Leo's own concern. If objects could consume Paramecia or Logia-type Devil Fruits, could they create magical items?
Stussy sighed and said, "Doctor, while this is a great discovery, its practical use is very limited. I don't think it's worth further research."
This highlighted the difference between Vegapunk and the others. Vegapunk pursued science relentlessly, a true pioneer. Judge cared about technology that served his ambitions, focusing on bloodline factor transfer, and was frustrated by Vegapunk wasting Devil Fruits. Stussy valued practicality—she saw a sword turned into a Devil Fruit weapon as little more than a toy. Caesar and Queen, meanwhile, weren't interested at all.
As everyone discussed the Bull Sword, Leo whispered, "Perhaps there's a way to make this research more practical." Suddenly, everyone quieted and looked at him.
"You could try having other objects consume fruits—like giving a cloak a flying-type Devil Fruit so it could carry someone. If we keep thinking along these lines, a person could potentially possess multiple Zoan abilities. There's another possibility too. Have you heard of ship spirits? Ships are said to hold souls. If a ship spirit consumed a Paramecia or Logia-type Devil Fruit, what would happen? Perhaps we could obtain a ship with superpowers."
Leo's words left the others staring blankly at each other. They had to admit—he'd proposed a much broader idea. After a moment, Vegapunk spoke: "Your second hypothesis has too strict a verification condition, so we'll set it aside for now. The first hypothesis is feasible, as long as we choose the right Zoan-type fruit." The others continued discussing Leo's idea but ultimately decided to temporarily shelve it.
After this discussion, Leo had clearly made an impression as a sharp thinker. Thankfully, in later interactions, everyone began valuing his opinions.
Life returned to normal. Assistant work required skill, and Leo gradually adapted, freeing up plenty of time each day. He finally had time to study what he wanted. Since Vegapunk had shown him the transformable mechanical armor, Leo began dreaming of developing his own vehicle.
However, Leo's mecha needed to integrate Devil Fruit powers, requiring a redesign. This was Leo's original idea. Once he saw the blueprint, he decided to base the system on fruit abilities enhanced with technology to achieve peak combat power in the pirate world.
One day, Leo sat at the workbench, manipulating a constantly changing water orb with one hand while carefully drawing blueprints with the other. He had dismantled Vegapunk's transformable mecha, studied every component, and began designing his own.
Later, Vegapunk entered the lab, glancing at Leo and frowning: "If you want to draw, focus. This isn't playtime."
Leo paid no mind: "Doctor, I'm not playing. I'm training my water-control ability. I want to simulate the mecha using seawater."
"Queen's right. You're a heretic. Machines of gears and metal are a man's romance. What's the point of simulating one with your ability? Besides, a mecha made of water—will it even work?"
Leo didn't argue, merely smiled slightly: "Doctor, watch this." Two streams of water surged from the tank beside him, merging with the water in his hands. The color gradually shifted from light blue to deep blue, eventually glinting with metallic sheen.
He gestured for Vegapunk to check. The doctor rushed forward, astonished: "This is… Sea-Prism Stone? And the purity is high. How did you do it?"
Leo beamed: "I used my Devil Fruit ability. Seawater contains trace Sea-Prism Stone factors. I can extract, concentrate, and solidify them into any shape I want."
"So you can replace metal with Sea-Prism Stone? No wonder you practiced simulating mecha parts with seawater. Your ability is incredible—I thought you were Logia-type."
"I can't become an element," Leo said, waving his hand, and the Sea-Prism Stone ball floated back to him.
Half a month later, Leo successfully simulated all mecha components and began assembly. Once complete, he transformed the seawater mecha into Sea-Prism Stone using his powers.
When finished, Leo nearly jumped with excitement. It was unbelievable. If Vegapunk had made a portable Iron Man MK5, Leo's fruit-powered mecha was essentially the Iron Man MK50—super nanotech armor.
But that wasn't the end. Vegapunk's laser cannon was stronger than Leo's due to charging differences. Leo needed a more efficient energy source.
He thought of Iron Man's Arc Reactor—a stable, high-efficiency power source. But the problem: he didn't know how to build one. He had to ask Vegapunk for help.
"What's an Arc Reactor? Where did you get such strange knowledge?"
Leo recalled every detail and explained the design—mentioning rings, vacuum, magnets, coils, and more. After Vegapunk fully understood nuclear fusion, he began experiments.
The One Piece world had a wildly polarized tech tree: from black powder cannons to futuristic devices. Leo expected the Arc Reactor to take ages, yet Vegapunk completed it in under a week. When the glowing energy core was handed to Leo, he was stunned.
"Did you really make this?"
"Hmph, it's not complicated. Besides, it resembles technology left by a great kingdom."
"Giant Kingdom?" Leo was surprised Vegapunk knew of it.
"Forget I said anything. Stop talking nonsense—install it and test it."
Following Vegapunk's advice, Leo formed the Sea-Prism Stone armor again and slotted the energy core into its chest.
His eyes sparkled: "Doctor, I'll become the Tony Stark of the Grand Line."
"Tony Stark? Who's that?"
"Uh… a top scientist and a playboy genius."
"Impossible!" Vegapunk jumped like a cat whose tail had been stepped on.
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All the fanfiction is 30-chapter ahead but this "One Piece: Shadows of the Throne" is 30 chapters ahead there