One possible plot is that there's a mysterious force threatening the Vocaloid world. Hatsune Miku and Kagamine Len are the only ones who can stop it. They have to combine their powers, which are related to their music abilities. Miku's ability to reach high notes and Len's ability to create catchy melodies could be the key to defeating this evil force. During this journey, they face many challenges and their relationship evolves.
One common theme is love and romance. Writers often explore the relationship between Len and Miku in a romantic context, creating sweet and sometimes angsty love stories.
You can start by looking on fanfic websites like Archive of Our Own (AO3). There are a lot of Vocaloid fanfics there, and you can specifically search for ones related to Miku and Len. Fanfiction.net is also a great place. Just use the search bar and type in 'Hatsune Miku x Kagamine Len' and you'll get a list of relevant fanfics.
You can start by looking on fanfiction websites like Archive of Our Own (AO3). It has a large collection of fan - created works, including many about Miku and Len. Just use the search function and type in their names.
There's also 'A Duet in the Digital Realm'. In this fanfiction, Miku and Len are digital avatars who discover a hidden part of their virtual world. They have to rely on each other to explore it, and in the process, their feelings for each other deepen. It's a really interesting take on their relationship.
You can try searching on fanfiction websites like Archive of Our Own (AO3). There are often a large number of fanfics about Len and Miku there. Just use their names as keywords in the search bar.
Another theme is music - making. Since they are both virtual singers, their combined efforts in creating music are often a central part of the stories. Some fanfictions might show them collaborating on a new song that has the power to change the virtual world for the better, or they might be in a competition to create the best song.
Their relationship can be developed through shared musical experiences. For instance, they could start by collaborating on a single song. As they work on the lyrics, melody, and production, they get to know each other better. Their different musical tastes can either clash at first and then gradually blend, or they can immediately find harmony in their differences.
I've heard of a fanfiction called 'The Songbird Duet'. In this story, Miku and Rin are on a quest to find the rarest musical note. Along the way, they learn more about each other's insecurities and strengths. Their friendship grows deeper as they face various obstacles in the search for this special note, which is said to hold the key to ultimate musical harmony.
One great Kagamine Rin x Len fanfic is 'The Melody of Their Bond'. It beautifully portrays their relationship through a series of adventures in a magical music - filled world. The story has a great balance of cute interactions and deeper emotional moments between the two.