I'd recommend checking out 'The Adventures of Nepeta and Equius' on Archive of Our Own. It has a great plot that really brings out the best in both characters.
One of the great things about Nepeta x Equius fanfics is the potential for character growth. We might see Equius becoming more in touch with his emotions through his interactions with Nepeta. And Nepeta could also learn about boundaries and self - control from Equius. Fanfic writers can take these two characters in many different and creative directions in their stories.
One main theme is the clash of personalities. Equius is so rigid and proper while Vriska is more wild and unpredictable. Their relationship is often about how they deal with these differences.
A prevalent theme is the idea of redemption. Sometimes Sollux or Aradia might have something they need to make up for, and their relationship helps in that journey. Also, the theme of adventure is quite common. They might go on quests together, exploring new areas in the Homestuck universe, which tests and strengthens their bond.
One common plot is their exploration of different Alternian cultures together. Since they come from different backgrounds on Alternia, fanfictions might delve into how they share and learn from each other's cultural experiences.
There's a fanfic called 'Equius and Vriska: A New Understanding'. In this, it starts with their typical rivalry but then gradually shows them finding common ground. It's a great read if you like seeing their relationship evolve in unexpected ways.
Archive of Our Own (AO3) is a great place to start. It has a large collection of Homestuck fanfictions, including many about Eridan and Aradia. You can use the search function to specifically look for their pairing and then sort by kudos or bookmarks to find the more popular ones.
Sure. One popular Sollux x Aradia fanfic is 'A Meeting of Minds'. It beautifully explores their relationship through various shared adventures in the Alternian setting. Another great one could be 'The Bond Between'. It delves deep into how their personalities complement each other and how they grow together in the story.
One common theme is friendship. Their bond is often explored, like how they understand each other in ways others might not. Another theme can be adventure, as they go on various quests together in some fanfics.