One idea could be Luffy using his intelligence to outwit powerful enemies. For example, instead of just relying on his strength in a fight against a devil fruit user with tricky abilities, he figures out their weakness through careful observation and then uses it to his advantage. He might also use his smartness to form strategic alliances with other crews or kingdoms in the One Piece world.
Another idea is that in the fanfiction, Luffy's intelligence is related to his ability to understand ancient languages or prophecies. This could lead him on a new adventure to uncover hidden secrets in the One Piece world, like the true meaning behind the poneglyphs. He could then use this knowledge to get closer to the One Piece.
Maybe in the fanfiction, a smart Luffy figures out a way to read Poneglyphs with Robin's help. This new ability draws the attention of both the World Government and other pirate crews. They then have to go on a journey to protect this new - found knowledge while also using it to uncover more secrets about the Void Century.
One interesting Smart Luffy fanfiction could be where Luffy uses his new - found intelligence to outwit powerful enemies in a strategic way. For example, instead of just charging into battle, he devises elaborate plans that take into account the enemy's weaknesses.
It could be a super intelligent pirate who has been secretly controlling events in the One Piece world. This antagonist might be several steps ahead of everyone, creating elaborate plans to take over the Grand Line. Luffy, with his newfound intelligence, has to match wits with this foe.
The contrast it creates. Usually, Luffy is seen as a bit of a goofball, so when fanfiction makes him smart, it's a fresh take.
Perhaps a plot where Natsu, in a fan fiction, is given the task of training a group of young wizards who have lost their way. He has to use his own experiences and his smart, godly nature to inspire them. This could involve him taking them on adventures where they face challenges that force them to grow. At the same time, Natsu has to deal with an evil wizard who is trying to corrupt the young wizards for his own gain.
One interesting idea could be exploring new abilities Luffy discovers within Gear 5 that he hasn't shown in the official story yet. For example, he might be able to manipulate his surroundings in even more creative ways, like turning the air into rubbery substances to create platforms or traps.
One idea could be that Luffy, being smarter, strategizes better during battles. He might analyze his opponents' weaknesses more quickly and come up with unique battle plans instead of just relying on his brute strength. For example, against a devil fruit user with the power to control water, he could figure out a way to use the environment to his advantage, like leading the fight to a place where there's less water or finding a way to nullify the water's effect on him.
One idea could be to create a fanfic where Luffy and his crew end up in a parallel universe. In this universe, the Straw Hats have different abilities and they have to figure out how to use them to get back home. Another idea is a time - travel fanfic. Luffy travels back in time and meets Gol D. Roger. He could learn some valuable lessons from Roger and use that knowledge to strengthen his crew.
One idea could be a Luffy crossover with Naruto. Luffy's carefree and determined nature could contrast with Naruto's own strong - willed personality. They could team up to face a common enemy that threatens both their worlds, like a powerful demon that can travel between dimensions. Luffy's rubber abilities and Naruto's ninja skills would make for some really cool fight scenes.