One potential character could be an old friend of Luffy's mother from her celestial dragon days. This friend might have different views now and could either help or hinder Luffy depending on their own motives. Another could be a servant who knew about Luffy's mother's secrets and has been waiting for the right time to reveal them to Luffy.
A possible character could be a former ally of Luffy's mother who has turned to the dark side. Maybe they were once fighting together for a better world but then got corrupted by power. This character would be a formidable enemy for Luffy as they know a lot about his mother and could use that knowledge against him. And there could be a young celestial dragon who looks up to Luffy because of his mother's legacy and wants to follow in her footsteps to change the celestial dragon society.
There could be a rival celestial dragon who knew Luffy's mother. This rival might be jealous of her relationship with someone outside the celestial dragon world or her attempts to change things. They could be a constant threat to Luffy, trying to stop him from achieving his goals just because of his mother's past. Also, Luffy's grandfather might have a more complex relationship with Luffy's mother as a celestial dragon, and this could be explored in the fanfiction.
Well, one possibility could be that Luffy's mother was a celestial dragon who rebelled against their cruel ways. She might have fallen in love with Luffy's father during her journey of self - discovery away from the celestial dragon lifestyle. And then she had to protect Luffy from the wrath of the other celestial dragons as he grew up with his dreams of freedom.
There could be some of Dragon's comrades in the revolutionary army who become important figures in Luffy's upbringing. For example, a veteran revolutionary who teaches Luffy about the history of the world and the wrongdoings of the World Government.
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.
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.
Potentially, we could see some celestial dragon characters. They might be ancient and powerful beings. Some could be friendly towards Natsu, seeing him as a new hope for their kind. Others might be more hostile, seeing him as an intruder or a threat to their traditional ways. There could also be other mages from different guilds who are either curious or envious of Natsu's situation with the celestial dragons.
In 'Luffy Lemon Harem Fanfiction', Nami is an obvious choice. She's always there to scold and care for Luffy at the same time. Robin, with her intelligence and elegance, could easily be part of it. There could be a girl from a sky island. Since Luffy has this amazing ability to make friends and influence people, this sky island girl might be fascinated by his adventures and fall for him. Additionally, a female marine who has a change of heart after seeing Luffy's selfless acts could also be in the harem.
One potential main character could be an older Celestial Dragon who has seen the worst of his kind's actions over the years and decides to make amends. He could be a very complex character, torn between loyalty to his family and his new - found sense of justice. Then there's the possibility of a Celestial Dragon - turned - pirate. This character would have a unique backstory, having grown up in luxury but choosing a life of adventure and freedom on the seas. Also, a commoner who somehow gets involved in the affairs of the Celestial Dragons, perhaps by saving one's life and then getting dragged into their world of power and politics, could be a main character.
After Luffy kills a Celestial Dragon in fanfiction, the story might see the emergence of new allies for the Straw Hat Pirates. There are likely many characters in the One Piece world who secretly despise the Celestial Dragons and would be inspired by Luffy's bold move. These new allies could bring new skills and resources to the crew. Also, the World Government might start to show its cracks as internal disputes could arise over how to handle Luffy. Some might want to go all - out to destroy him, while others might see the potential for a change in the current power structure.
One possible plot could be Luffy using his newfound Celestial Dragon status to infiltrate the highest echelons of the World Government. He could pretend to be on their side while secretly working to expose their corruption and free the oppressed. For example, he might gain access to classified information about the Void Century and use it to change the world view.
Igneel could also play a role. Since Igneel was Natsu's adoptive dragon father, he could appear in Natsu's mind during the transformation, guiding him and giving him advice on how to handle his new dragon powers. And there could be other dragons in the story as well. They might be either friendly or hostile towards Natsu depending on their nature.