In a plotline, Luffy could find himself in a strange and magical island where Ace is being held. The island has its own rules and obstacles. For example, there could be illusions that try to mislead Luffy. But Luffy, with his unwavering belief in saving his brother, overcomes these illusions. He might also meet some friendly locals who help him understand the secrets of the island and finally reach Ace. Then he'd engage in a fierce battle with the captors to free Ace.
A possible plotline is that Luffy discovers that Ace is being held in a heavily fortified prison in the sky. Luffy has to find a way to reach the sky prison first. He might team up with some sky - dwelling creatures or pirates who know the way. Once there, he has to fight through multiple layers of security, including guards with special powers. Luffy's growth as a pirate and his love for Ace would be on full display as he battles his way through to save his brother.
He might face powerful enemies. Maybe a new pirate crew with a devil - fruit user who can nullify Luffy's powers. Luffy would have to find a way to overcome this power - nullifying ability to reach Ace.
One possible plotline could be that Luffy gets into a really tough battle against a powerful enemy that he can't defeat on his own. Dragon, being his father, senses the danger and arrives just in time. He uses his mysterious powers, like controlling the weather, to turn the tides of the battle in Luffy's favor. Then, he might give Luffy some words of wisdom about his journey and the importance of his goals before disappearing again.
One possible plotline could be that Harry discovers a hidden pirate ship in the Forbidden Forest. He then decides to join the crew and learns the ways of piracy. Another could be that he is magically transported to a pirate world and has to adapt to survive as a pirate.
One possible plotline could be that the princess is from a wizarding kingdom that has been taken over by dark forces. Harry, being the hero he is, stumbles upon the situation during one of his adventures. He uses his magic skills, like the Patronus charm to fight off the Dementors that are guarding the princess. Then he and the princess have to work together to find a powerful magical artifact that can restore her kingdom.
In some fanfictions, character corruption can be a factor. The author might decide to corrupt the pure relationships in the original story. So Ace and Sabo are written as hating Luffy because they are influenced by some external dark force or bad ideology within the fan - created universe.
One possible plotline could be that a character from Ace Attorney is transported to the world of Aladdin and has to use their legal knowledge to solve a problem there. For example, they might have to prove the innocence of Aladdin in a court - like setting in the magical world.
One possible plotline could be that Naruto is in a life - threatening battle against a powerful enemy. Gaara arrives just in time, using his sand powers to shield Naruto from a fatal attack. Then they team up, with Gaara's strategic thinking and Naruto's unwavering determination, they gradually turn the tide of the battle.
One possible plotline could be that Kumo is under attack by a powerful and mysterious enemy. Naruto, being the hero he is, hears about it and rushes to save the day. He might use his newfound Sage Mode powers or his ability to connect with the Nine - Tails to fend off the attackers.
One possible plotline could be that Sakura is in a dangerous battle against a powerful enemy. Madara, who might be an unexpected ally, steps in. He uses his powerful jutsu to fend off the enemy and saves Sakura. Maybe Madara has his own reasons, like seeing Sakura's potential or having a newfound respect for her skills.
One possible plotline could be that Naruto arrives just in time to stop Haku from sacrificing himself for Zabuza. He uses his shadow clone jutsu to create a distraction and then manages to pull Haku out of harm's way. Naruto then tries to convince Haku that there is more to life than being a tool for Zabuza.