One popular fem Luffy and Ace fanfiction might be 'Sisterly Bonds'. It delves into their relationship with a new perspective of Luffy being female. It shows how their interactions change and yet still hold the essence of their brotherly (in this case, sisterly) love in the original story. Another could be 'Fem Luffy's Adventure with Ace', which focuses on their adventures together with a unique take on fem Luffy's character development in relation to Ace.
I would recommend 'Ace, Luffy and Sabo: Reunion of Souls'. This fanfiction is full of heart - warming moments. It shows how their past experiences shape their present interactions and how they support each other in the face of difficulties.
First, you need to really understand the characters of Luffy and Ace from the original One Piece. Think about how Luffy's character traits would be different or the same if she was female. For Ace, consider how his relationship with a female Luffy would develop. Then, come up with an interesting plot. It could be an adventure they go on together, or a situation where they have to face a common enemy. Also, don't forget to add some emotional depth to their relationship.
One common theme is the strong emotional bond between Ace and Luffy. It often explores their relationship in a more romantic or deep - seated way compared to their canon friendship. Another theme could be adventure, as they are both characters from One Piece, so the fanfics may incorporate the elements of their pirate adventures while also delving into their relationship.
I'm sorry, but I don't have specific ace x luffy yaoi fanfic to recommend directly. You can try searching on some popular fanfic websites like Archive of Our Own (AO3). There are likely many fan-created works there related to this pairing.
Adventure is also common. They are pirates after all. Fanfictions might take them on new quests or have them explore uncharted islands together, much like in the original series. For example, they could be searching for a hidden treasure that has a special meaning to their family. It's a way to continue the spirit of adventure that One Piece is known for in a new context.
The popularity also stems from the fact that there are so many unexplored aspects of their relationship in the official story. Fanfiction allows writers to fill in those gaps and create stories that satisfy the fans' curiosity. For example, their early days together or how they would interact in different settings.
Archive of Our Own (AO3) is a great place to start. It has a large collection of fanfictions, including many for 'ace x luffy x sabo'. The search function there is quite effective in helping you find the ones that suit your taste.
One good 'One Piece Ace x Luffy fanfiction' is 'Brothers in a New Light'. It explores their relationship in a different AU (alternate universe) where they face new challenges together. The story focuses on how their bond remains unbreakable even in the face of unexpected situations.
The theme of acceptance is quite common. Fem Luffy often has to be accepted by the other characters in the story. This could be because she's different due to her gender or her slightly different personality traits. Also, the theme of self - discovery is prevalent. As fem Luffy navigates the pirate world, she discovers more about herself, her abilities, and her place in the grand scheme of things.