One idea could be that Luffy accidentally discovers a secret about a Celestial Dragon that could change the power balance in the One Piece world. He then has to decide whether to expose it or use it to his advantage.
In a Luffy Celestial Dragon Fanfic, the class struggle can be shown through Luffy's interactions with the Celestial Dragons. Luffy, who comes from a humble background, can represent the common people. His disregard for the Celestial Dragons' status and power can be a way to highlight the unfairness of the class system.
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.
Yes, in some fanfictions, Luffy can be more like Dragon. Luffy and Dragon both have a strong sense of justice. Luffy wants to become the Pirate King and free the oppressed islands, which is similar to Dragon's goal of overthrowing the World Government to create a more just world. Also, they both possess great leadership. Luffy leads his crew, and Dragon leads the revolutionary army.
Well, one possible plot could be that Luffy, while on one of his adventures, accidentally stumbles upon Dragon in a secret island. They initially don't recognize each other, but then some events unfold that make them realize their connection. Maybe they end up having to team up to fight against a common enemy that is threatening the freedom of the island's inhabitants.