First, focus on the characters. Develop their personalities, backstories, and relationships in depth. For example, if it's a fanfiction about a couple from a TV show, think about how they would interact in different situations outside of what's shown on the screen.
Use dialogue effectively. The conversations between the couple should sound natural and help to move the story forward. It can also reveal their feelings for each other and their individual personalities. Avoid using cliched phrases and try to make the dialogue unique to the characters. By following these steps, you can write a great couple - centered fanfiction without any inappropriate content.
Create an interesting plot. It doesn't have to be overly complicated, but it should have a clear beginning, middle, and end. Maybe the couple goes on an adventure together, or they face a challenge that tests their relationship. Also, use vivid descriptions to bring the story to life. Describe the settings, the characters' emotions, and their actions in detail. This will make the fanfiction more engaging for readers.