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.
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.
A possible plotline could be a battle scenario. Maybe there's a powerful enemy threatening both Luffy's crew and Marco's group. So they meet on the battlefield and have to quickly figure out how to work together. Their different fighting styles, Luffy's Gum - Gum powers and Marco's phoenix - related abilities, make for an exciting combination in combat.
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.
Perhaps there could be a plot where an evil force is threatening both the ninja world and the pirate world. Female Luffy and Naruto are chosen as representatives of their worlds to form an alliance. They have to combine their skills, with Naruto's ninjutsu and female Luffy's Gum - Gum powers. As they travel together, they also develop a deep friendship, and the story could explore how their different cultures and values interact and influence each other.
Well, 'Luffy and Ace: Unbreakable Ties' is a fanfiction that I really like. It focuses on their childhood memories and how those shaped their relationship as they grew up. There's also 'Ace and Luffy's Great Escape' which is full of action. It shows their cleverness in getting out of difficult situations. 'Luffy and Ace: The True Friendship' is a must - read too. It simply and effectively shows the depth of their friendship.
It could be a time - traveling version of Blackbeard. Maybe he also discovers time travel and uses it to his advantage to stop Luffy at every turn. He could go back in time and try to prevent Luffy from getting important allies or Devil Fruits.
One possible plotline could be Apollo taking on a case that seems impossible to win at first. He has to dig deep into the evidence and find some overlooked details to turn the case around, just like in the original game. For instance, he might discover a hidden witness or a crucial piece of evidence that was misinterpreted.