Often, the theme of longing and desire is prevalent. Since their relationship has so many obstacles in the original story, fanfictions tend to play with the idea of their unfulfilled desires. This can be seen in how the lemon elements are incorporated to heighten the sense of yearning between them. Additionally, themes of jealousy and possessiveness might also come up, as both characters have strong personalities and emotions when it comes to each other.
You can try searching on fanfiction platforms like Archive of Our Own (AO3). It has a large collection of fanfictions, and you can use the search bar to specifically look for works related to Nana Osaki and Ren Honjo. Another option could be FanFiction.net, although the content quality may vary more there.
One popular 'nana x nana fanfic' story could be about their adventures in a new city together. They might discover new music and bond over it, exploring the local music scene.
First, understand the characters well. Know their personalities, dreams, and fears. For the 'nana x nana' pairing, it's important to capture their unique bond.