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.
The Gamzee x Karkat comic has a story that keeps you on the edge of your seat. It combines drama, adventure, and a touch of mystery. The relationship between the two main characters is at the core of the narrative, creating a compelling read.