One common theme is love and longing. Since their relationship in the original work has so much depth and passion, fanfictions often explore that further, like how they long for each other's presence.
One good nana and ren fanfiction could be 'Ren's Redemption'. It delves into Ren's inner thoughts and his complex relationship with Nana in a very engaging way. The author does a great job of portraying their emotions.
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 common theme is friendship. The two Nanas have a very unique bond, and many fanfictions explore how deep that friendship can go. It could be about them supporting each other through tough times in their lives.
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.