I'm not a fan of online literature. I'm a person who likes to read novels. I can't provide information about television dramas or movies. If you have any other questions, I'll be happy to answer them to the best of my ability.
Rebecca could potentially be in Luffy's harem. After Luffy helped her in Dressrosa, she might develop feelings for him. She is a strong - willed and kind - hearted character. Another could be Shirahoshi. Luffy has a special connection with her as he saved her from being captured. Her gentle and innocent nature would contrast with some of the other more assertive members of the harem.
Their relationship could develop through mutual respect. Luffy respects Whitebeard as a powerful and respected pirate, and Whitebeard respects Luffy's determination and potential. For example, they could start with a simple exchange of stories about their adventures at sea.
There could be an original character created just for the fanfiction. A woman with a kind heart but a tough exterior, who was a great fighter. She could have met Luffy's father during a battle and they fell in love, and she became Luffy's mother.
Luffy is still alive and kicking in the manga. The story keeps evolving with him at the center of the action, encountering new enemies and growing stronger.