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After a lot of chaos, punching, crushing buildings, punching, encounters, and more punching, Luffy and his nakamas were able to make it to Fish-man Island and return safely. Apart from the New Fish-man Pirates' plotting to overtake Ryugu Kingdom and make Fish-men be recognized as the supreme race, they were able to meet Fisher Tiger, the ex-captain of the Sun Pirates and learned that Natsu saved him years ago from the Marines.
Despite Natsu telling him that it was not necessary to treat Luffy well just for an action that cost him nothing, Fisher Tiger felt that he should at least show kindness to his savior's youngest brother.
The rest happened more or less as in the original story and Luffy asked for Jinbe to join his crew. However Jinbe says that he wants to stay in the reconstruction of the country and says that for the moment he cannot join him, but when they meet again he will join.
Fisher Tiger immediately realized how powerful Luffy was. Not just physically, but that ability to attract people.
That reminded him of Natsu, who basically has half the world as his friends. From Yonkos like Whitebeard and Shanks to Marines like Kizaru, Garp and even Akainu, despite the fact that the latter denies it with all his being.
(His eyes don't deceive him. He saw how Akainu interacted with Natsu when they were drinking together in a bar. Even though the older man refuses, he sees Natsu as a good friend.
A strange but stable friendship.)
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Luffy was looking at the wanted posters that Zeref gave him before he left Amazon Lily.
He completely forgot to look at it and only remembered it when he wanted to see Jinbe's new poster.
Zeref is someone who pays attention to details, compared to Natsu. He made sure to give Luffy all the wanted posters of all the pirates worth taking a look at.
Amongst that bunch of posters Luffy noticed a specific poster.
"Black Widow" Charlotte Katakuri.
Reward: 1.057.000.000 bellies.
-Wow... That's a lot of reward.- Vivi comments, looking at the wanted poster.
-Huh... I would like to meet her...-
-... Don't tell me you want to fight her...- Vivi was pale and began to imagine the worst scenario possible.
-Hmmm... Maybe...- Luffy mutters thoughtfully as he continued to look at the poster.
-For God's sake... Luffy... Don't do it. We will end up dead!!!-
-But I want to!- Luffy pouts.- I'm strong!!-
-But she is stronger!!! That without counting Big Mom!!! A fucking Yonko!!!-
Vivi and Luffy started arguing. Vivi was trying to convince Luffy that going for a Yonko is still too risky while Luffy reassured her that nothing was wrong. Later, more Straw Hats joined the discussion. Some stayed neutral while some supported Vivi and others supported Luffy.
This discussion ended with Luffy's victory. Arguing with him about something that he already planned to do is completely useless, they all learned it a long time ago so they decided to change their course towards Totto Land, but their plans were thwarted when a call came from Law.
(During the trip, Luffy was so distracted that even Chopper realized something was wrong with him. They don't know why and it worries them, but they were busy with what's going to happen in Dressrosa.
They thought that it could be that Luffy for once is worried about the enemy they are going to face soon, as unlikely as it is.)
(Robin watched Luffy's expression for a long time and a small light passed through her eyes.
Perhaps...)
Luffy wouldn't say no if Law asked him for help. He would never turn his back on a friend and he knows that Law has hated Doflamingo for 13 years, not because he did something terrible to him, but he did hurt Rosinante.
What happened on Minion Island it wasn't a big deal, just a beating from Vergo. What hurt Rosinante was what Doflamingo did afterwards when Natsu managed to save them.
Doflamingo, once he got the title of Shichibukai, the first thing he did was set a bounty to KILL Rosinante. That was what hurt him the most. This showed that if they hadn't escaped and Doflamingo had found them, neither of them would have survived.
Law will never forget the numbers of people from the North Blue who tried to kill Rosinante because of this.
Seeing how sad the blond looked every time something similar happened made his blood boil.
Doflamingo has to pay for the damage he is causing not only in Rosinante, but in Dressrosa as well. To this day, Rosinante regrets not notifying Sengoku in time of Doflamingo's plan to carry out a coup in that country. He felt that it was his fault that the citizens there is suffering. Rosinante wanted to try to help them somehow.
And the easiest way to do that is by defeating Doflamingo.
Law is more than aware that he alone could not defeat Doflamingo so he asked for help. Ace obviously wasn't going to let her boyfriend fight that psycho bird alone so she dragged Sabo over to help. Sabo managed to convince Smoker to help free Dressrosa and she brought Tashigip.
Law thought about it a lot and decided that Luffy was strong enough to participate in this, plus Luffy is a magnet for chaos just like his siblings. Once the ASL siblings get together then it's not something as easy as 1 + 2 = 3... He is sure that he will enjoy the pain that Doflamingo will feel once he has these three as an enemy at the same time.
But before they face the king... They must deal with a certain mad scientist and the first Corazon...
Law will make sure to pay him back for all those hits he and Rosinate took 13 years ago.
(Law literally had a little notebook in his mind where he recorded all the bad things other people did to him so he could get revenge in the future.
He is spiteful and loves revenge. If someone doesn't like that then he will just give them the middle finger and say "Fuck you".)