One common theme is the exploration of their contrasting personalities. Rei is often portrayed as more reserved and introspective, while Jadeite can be seen as more bold and assertive. Their relationship can be a study in how these different personalities interact and complement each other.
Another theme could be the idea of redemption. If Jadeite has some not - so - good actions in the original story, fanfics might explore how Rei could be a factor in his redemption arc. It could be about how their relationship helps him become a better character. Also, themes of forbidden love can be present, as they might be from different sides or have different allegiances which makes their relationship a bit of a taboo in the context of the story world.
Often, there are themes related to power dynamics. Rei may have certain powers and Jadeite his own, and how these interact within the context of their relationship can be a theme. Also, themes of redemption might be present, especially if Jadeite has done some not - so - good things in the original story. And of course, the theme of love and attraction between them is quite common in such fanfiction.
Some rei x jadeite fanfics might explore their relationship in a more angsty way, perhaps dealing with the differences in their personalities or their loyalties in the context of the original story they are from. Others could be more light - hearted and focus on their interactions in a more comical or romantic setting. You really have to dig in and read a few to find the ones that suit your taste.
Another theme is mutual support. Saeko and Rei often support each other in different situations. In some fanfics, Rei might help Saeko with her emotional struggles, while Saeko could be there for Rei when she needs physical protection.
One common theme is friendship. Since they are characters from the same series, their friendship is often explored in depth. Writers might show how they grow closer over time.
One common theme is the 'forbidden love' aspect. Since Jadeite is often an antagonist in the Sailor Moon universe and Usagi is the protagonist, their relationship goes against the natural order, which makes for an interesting and often angsty story. Another theme could be 'redemption'. Jadeite might be shown as trying to redeem himself through his relationship with Usagi.
One common theme is friendship. Rei and Usagi have a strong friendship in the original series, and fanfictions often expand on that. They might go through challenges together and come out stronger.
One common theme is the exploration of their relationship. This could involve them falling in love, dealing with misunderstandings in their relationship, or growing closer over time. Another theme is adventure. Since they are often in some sort of action - oriented world, fanfiction might focus on their joint adventures, fighting against enemies and overcoming obstacles together.
One common theme is the development of their relationship. It could be about how they first meet and start to have feelings for each other. Another theme might be adventure. They could be on a journey together and their relationship deepens during the adventure.
Love, either platonic or romantic, is also a frequent theme. Sometimes it's a slow - burn type of love where they don't realize their feelings for a long time. Other times it's a more immediate connection that they have to navigate through various obstacles in the story.
One common theme is friendship. Their bond might be explored as they face various challenges together.