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.
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.
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.
She could be a former pirate captain herself. Maybe she was one of the most feared pirates in her time, and her skills and reputation were passed down to Luffy in some way, even if he didn't know it was from his mother.
Well, in such a fanfiction, Luffy might gain some valuable advice from Roger. Maybe Roger could share his experiences about the One Piece and the Grand Line, which would inspire Luffy further in his journey to become the Pirate King.
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.
Well, it could be that Luffy tries to recruit Whitebeard into his crew. Luffy is always on the lookout for strong allies, and Whitebeard with his immense power would be a great addition. However, Whitebeard might laugh it off, seeing Luffy as a young and brash pirate with big dreams but still a lot to learn.
In this fanfiction, Luffy might initially be in awe of Whitebeard. He could be eager to learn from the great pirate. Maybe they'll team up for a short adventure against a common enemy, like a powerful marine force. Luffy's energy and Whitebeard's experience could make for an interesting dynamic.