Another theme could be redemption arcs. Maybe Eridan has done something wrong in the story's context and Karkat helps him to become a better person. It can be a really interesting way to develop their relationship within the fanfiction.
You can try Archive of Our Own (AO3). It has a large collection of Homestuck fanfictions, including those about Eridan and Karkat. Just search for 'Eridan x Karkat' in the search bar and you'll get a bunch of results.
The fanfics usually have a lot of dialogue that is very characteristic of the two. Sollux's tech - speak and Eridan's more dramatic language can be used to great effect to show their individualities and how they interact with each other. It also often delves into their backstories and how those shape their relationship in the present.
Karkat is highly emotional. His passionate outbursts and strong opinions can create interesting dynamics in a 'Karkat x Reader' fic. He's not afraid to speak his mind, which can lead to both humorous and intense interactions with the reader.
One common theme is the exploration of their different personalities coming together. Eridan is often seen as more brooding and Gamzee as more chaotic, so their interactions can be an interesting study.
One common theme is friendship. Despite their differences, they often find a connection. For example, in many fics, they start to see each other's good qualities.
Adventure themes are also seen. They could be on a quest together in the Homestuck universe, facing challenges side by side, which then strengthens their bond and leads to a romantic relationship. This gives the authors a chance to explore their characters in new and exciting situations.
Another theme is the journey of self - discovery for both characters. The relationship can act as a catalyst for them to learn more about themselves. For example, the reader might help Karkat become more in touch with his emotions, and in turn, Karkat might help the reader become more confident. It's about growing together in the context of their relationship.
One great John x Karkat fanfic is 'A Strange Bond'. It beautifully explores their relationship as they navigate the challenges in their world. The author does an amazing job of portraying their personalities and how they grow closer over time.
One great 'Homestuck Gamzee x Karkat' fanfic is 'The Bond Between Them'. It really delves into their complex relationship, showing how their different personalities interact. The author does a wonderful job of portraying Gamzee's laid - back yet somewhat unpredictable nature and Karkat's more uptight and easily - flustered self. It's full of heart - warming moments as well as some comical exchanges that are true to the Homestuck spirit.