There are a lot of One Piece fan fictions around here, but most of them are left uncompleted. And here is the common mistakes One piece fanfic writers make often :-
First, they give the MC a huge training period from the beginning to raise their power to become acceptable, therefore missing on the golden opportunity to add some comedy to the story. Not just that, but they usually make the MC overpowered from the beginning, which makes the MC looks like an unmotivated bitch that just want to watch Luffy and his adventures without doing anything major.
Second mistake is sitting boundaries... If you ever gave your MC too much power, stop trying to stick with the main plot. There is almost no way for you to stick to the plot while not making your character boring. I saw this line said by almost every fanfic MC "I want to join straw hat crew, because I want to watch the series unfold with my own eyes" or something similar. Imagine having the knowledge of the plot, and the power to do whatever you like, but you always decide against telling people the plot because "I don't want to hurt their spirit" bullsiht. You should have typed "I am a lazy writer who can't adapt to adding a new character to the series". The thing is, an MC is called an MC, because the story is centered around him, not Luffy... ( I could talk about this issue for years, and I won't be able to give it its right ).
Third mistake. If you ever made the MC overpowered, and now you regret it, don't nerf him for no reason. It is a common mistake that a lot of authors do, even in other fanfics, not only one piece. If your MC became overpowered by a mistake, try to find entertaining ways to make him face a challenge, take Enel being unable to hurt Luffy for example. Give your MC someone that counter him, or if you couldn't find any, then don't stick to the original series and create a plot in which your MC is doing things that are entertaining, instead of laying back and doing nothing. If your MC is way too overpowered to the point that no one can stand a chance against him, then don't try to milk the fan fiction and prolong it; add comedic scenes instead of making it serious. Everyone would give you thumbs up if you slapped Akainu to another dimension, then went there and slapped him back to One Piece world.
Here is different story formulas that you can use...
If you wanted to write a story where the MC gets a certain amount of wishes from a god, write as follows:-
Write a backstory, and then you can say that the MC was hit by a truck... or if you want, you can make the most absurd reason of death. Just for the sake of comedy, lets say that Akainu was teleported by god by a mistake, and he slapped the MC too hard, that the MC's body popped and showered the whole road with blood.
Now you have established that the MC hates Akainu for killing him for no reason. Next, let the god offer you wishes and apologize as usual. Or you can make the MC very vicious and extort some benefits from god, up to you. Don't give the MC a lot of wishes, just give him 1 or 2. In the case of this example, I will let him have one wish.
During the anger of the MC, the MC will think of an ability that counters Akainu in theory, because his goal is to kill Akainu. For the sake of creativity I will choose (Shido Shido no Mi) also known as Temperature fruit.
Keep on mind of the following issues (Bodies in our world is way weaker than the bodies of One Piece world. Which means if you were to send your MC there with his own body, then even a baby can bitch slap your MC... I don't know how many people are overlooking this point... And the training excuse is just stupid. They don't have mana in the air there or anything that benefits the body, thus, training there will be the same as training here... How long does it take someone to be enlightened in this world? it takes decades of religious training to have a calm mind, let alone Haki and bullsht, so please stop sending your MCs with their original body to One Piece's world, because the only way you can argue against my point, is by saying that the world in there has more oxygen percentage in it's air, thus making creatures stronger, which can be countered by "A lot of oxygen means bigger body and higher metabolism, but we see a lot of characters in one piece eating like normal human beings")
Since you need the body from the one piece world, it is favorable to choose reincarnation. Being born anew as a child in the world of One Piece. If you choose to reincarnate him, make sure to mention that he was given birth to. You can choose whichever family you want.
As for the childhood, you can either skip it outlining what happened during that period, or you can use it to add some comedic value to the story by using a chapter or two to give context to the MC's childhood. For example, the sense of achievement that the MC felt when he started crawling, or walking! Or you can give him one line as a baby like :-
MC's mom "It is a good morning, isn't it (Insert MC's name here)?"
MC "Ga!"
Seagull extending his hand asking for money payment for the newspaper
MC "Ga!"
A fire burning the house, while everyone is panicking the MC is like "Ga!".
While it might sounds unfunny here, if done properly, it would be as funny as the peanut butter baby. (link in the comments XD)
It is better if you don't make your MC either strong, or weak, as he should be normal, as strong as a 1m berries pirate or something like that. If you wanted to continue the comedic scene, you can make your MC weak, and make fun of his weakness, but don't over do it, because no one likes a weak MC that has a weak character. Despite talking about how it is better to make your MC in the middle, not strong or weak, for this example I will make the MC overpowered after childhood, to assist those who fell into the mistake of making the MC too OP.
Lets say that our MC's weapons right now are as follows: -
1- The knowledge of the plot.
2- Mehh mastery over the temperature fruit ( can start fire on flammable objects, while freezing a bottle of water )
3- Introductory level in both Kenbunshoku and Bushoku Haki.
4- A strong body.
That is enough to make him strong enough to rival a 30 million berries bounty pirate, if not stronger.
Now, the MC have 3 things he can do, be a pirate, join the marines, or be a pirate hunter. The easiest early setup to develop an OP MC character is to make him be a pirate hunter, thus I am going to follow that example.
As the MC becomes a pirate hunter, (Insert a story here of how the MC got his boat or ship by either getting it as a gift from his family, or hunting a pirate that was terrorizing the people in his village and handing him over to the marine).
After the story of how the MC got his boat (Insert a story here of how the MC started hunting pirates, trolling them and destroying them easily cuz he is OP. example, make the MC light the pirates' ship on fire XD, and when they jump in the sea, he make the water around them cold, making them suffer from Hypothermia, dying a slow death by either burning or freezing).
An important thing that most of the people miss, is how the MC spends his money... You don't know how many authors talk about how the MC stole the treasures of the pirates, but never mention how the MC spent them. I am pretty sure their MC hid the treasures by putting it inside his ass!
As for ways to spend money, it contributes to the MC's character. You can make him spend it buying the best food ever, or saving it to buy a fake Zoan fruit in the black market, or maybe buying a high end products that will assist him, the same thing Usopp does with his money. You can use the money to bribe marines, hire pirate hunters, there are a lot of options because Money = Power.
The way the MC spends his money is also considered a part of his character, so don't be inconsistent of making the MC a man of justice, and then make him buy from the black market.
After the adventures of the MC, include shortcuts of people who are now paying attention to the MC.
As the MC is going to be OP (cuz of the example) he will have to train before every fight, thus having no problem destroying his enemies. If you want to extend your story, you can make the MC interact with characters of the main series, but don't make it too boring... A lot of chapters with no action might lead people to just drop your novel. For example, say that you want to troll Crocodile. Instead of killing him (and your MC can kill him easily because he will be OP when he meets crocodile), you can do a hide and seek game with him, by changing the weather using temperature (search more about the subject before writing about it, a google search will take you at most an hour). Ruining Crocodile's plan, will make him frustrated and he will act irrationally, then you can make your MC look cool by making fun of him in the middle of the fight.
The goal of the MC was to take revenge on Akainu; So you will have to align the events that occur so you can create opportunity for that to happened. As for how the MC can use temperature fruit to counter Akainu is very easy. Once Akainu turns to lava, the MC will freeze his lava into rock, and once Akainu converts back to his original form, he will be hit with a severe coldness that his bones will tatter. Remember, he heals himself with Lave, so everything he tried to heal, he won't be able to, because the cold temperature will cool his lava instantly (aka, the MC can control the temperature of the lava itself, making it sold in an instant, because the MC is OP)... and while you are at it, you can make it so the MC can make a space very hot that he can open portals between world, slapping Akainu to another world, and it will be known as the story where the MC is slapping Akainu across dimensions... Some say that Akainu is still being slapped to this day, stuck in a never ending cycle of being slapped through space and time.
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For those who want to make a fan fiction or the MC reincarnating without wishes, or the MC being born in the one piece world without the knowledge of the plot, or even the MC being reborn as a character, I will give examples for those, in the next chapter of Voidne's Books of Ideas!