They could start with a big hug. Then Shanks might tease Luffy about his still - crazy appetite. Luffy, in turn, could show off his new crew members to Shanks.
A powerful new pirate group could be the main antagonist. They might be after the same treasure or territory as Luffy and Shanks. They could have some unique and dangerous abilities, like the ability to control sea monsters. Their leader could be a former crewmate of a well - known and feared pirate, who has his own vendetta against both Luffy and Shanks for some past reasons.
One possible plot could be that Luffy, while on a new adventure in a mysterious island, accidentally discovers a hidden treasure map that leads to a place Shanks has been looking for. Luffy decides to find Shanks and share the map. When they meet, they team up to face the various pirates and obstacles in their way to reach the treasure. Along the way, Shanks teaches Luffy some new combat skills and they strengthen their bond as friends and pirates.
In this fanfiction, perhaps Luffy has become much stronger. He might show Shanks his new skills and powers. They could also talk about their adventures since they last met, like Luffy sharing his experiences on different islands.
Their relationship could develop through mutual respect. Ace respects Shanks as a powerful pirate, and Shanks respects Ace for his lineage and his own growing power. They might start as cautious acquaintances, but as they face challenges together, like battles against other pirates or dealing with natural disasters at sea, they grow closer and become friends who trust each other.
In fanfiction, when Shanks meets Luffy's crew, it could be a very emotional reunion. Shanks might be impressed by how much Luffy and his crew have grown. He could see the strong bonds within the crew and be proud of Luffy for leading them so well.
Maybe Ace is on a mission for the Whitebeard Pirates and Shanks is just sailing around. They meet at a port town. Shanks recognizes Ace's potential and tries to convince him to join his crew for a short - time adventure. Ace is hesitant at first but is intrigued by Shanks' laid - back yet powerful leadership style. They end up going on a treasure hunt together, with lots of banter and friendly competitions as they search for the hidden loot.
In the fanfiction, Luffy might show Shanks how much he has grown. He could display his new Haki abilities and his stronger crew. Shanks would be proud of Luffy's progress.
Blackbeard would probably be very interested in the magic in the Fairy Tail world. He might try to steal some of the powerful magic items or learn magic for himself. He could see the mages as a threat to his plans if they oppose him, but also as potential allies if he can manipulate them. He would likely use his cunning and power - hungry nature to his advantage in this new world.