One frequent theme is the power of their combined abilities. In fanfiction, their individual powers as Haru and Elie are often explored, and how when they work together, they can achieve great things. Also, the theme of sacrifice is there. For instance, one might sacrifice themselves for the other's safety, which is a very emotional and common plot point in these stories.
A common theme is adventure. Since they are part of the Rave Master story, many fanfictions have them going on quests and battles together. Another theme is friendship evolving into love. Their initial bond as comrades gradually turns into something more romantic as the story progresses in a lot of fan works.
I'm not sure which specific fanfiction is the best, but you can also try searching on Tumblr. Sometimes, fans post snippets or links to their own Haru x Elie fanfiction there. It might be a bit more scattered compared to the other two platforms, but you could stumble upon some really creative and unique takes on their relationship.
In 'elie and haru' fanfiction, one common theme is self - discovery. Both characters could be on a journey to find out more about themselves, and their relationship helps in that process. Also, the theme of loyalty often appears. They might show loyalty to each other in different situations, whether it's facing an enemy or making a difficult choice. There could also be a theme of growth, where they both grow as individuals and as a pair throughout the story.
Another common theme is power - up. Writers like to create scenarios where characters from both series gain new powers or enhance their existing ones. This could be through a combination of Fairy Tail's magic and Rave Master's unique abilities. For example, a character might learn a new spell by combining the power of a Rave Stone with their Fairy Tail magic. And also, the theme of adventure is quite common. As the two universes collide, there are new places to explore and new challenges to face, which makes for exciting adventure - based fanfictions.
One common theme is the crossover of power systems. Since Naruto has ninja abilities and Rave Master has magic-based powers, seeing how these interact is often explored. For example, a Naruto character might try to use chakra in the Rave Master world in a unique way.
One common theme is friendship turned into love. Since they are likely friends in the original story, fanfiction often explores how that relationship deepens into romantic love.
A common theme is friendship - turned - love. They start as friends and gradually develop deeper romantic feelings. Another theme is mutual support. They help each other through tough times, like Haru being there for Ryuji when he has family issues or Ryuji giving Haru confidence in social situations.
Often, there are themes of adventure. Haru might go on adventures with water as a sort of guiding or powerful force. For example, he could be exploring underwater worlds or using water to overcome obstacles in a magical journey. This type of theme can make for very exciting stories.
One common theme is friendship evolving into something more. Writers often explore how Kazu and Haru's relationship deepens over time, from just being friends to having romantic feelings. Another theme could be adventure. They might go on adventures together and through those experiences, their bond gets stronger.
One common theme is growth. They often start as two very different individuals, and through their interactions, they grow both personally and in their relationship. For example, Hibari might be more reserved and Haru more outgoing, and they learn from each other's traits.
I really like 'Haru and Elie: Unlikely Bonds'. This fanfic focuses on the unique connection between them. It has a lot of heartwarming moments where they support each other. The author does an excellent job of developing their characters within the story.