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.
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 a fanfiction where Luffy meets Roger, it could be an amazing encounter. Roger might see in Luffy the same spirit of adventure and freedom that he had. He could pass on some hidden knowledge about the One Piece or give Luffy a cryptic hint about the true nature of the Grand Line. This meeting could also inspire Luffy even more, fueling his determination to become the Pirate King. It would be a moment full of emotion and significance for both characters and for the entire One Piece universe.
It could be a powerful pirate crew that has a vendetta against either Luffy or Sabo. Maybe they have some history with the Straw Hat Pirates or Sabo's group.