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.
Yes, there are some. You can start by looking on fanfiction websites like Archive of Our Own. Just search for 'takashi x rei' and you'll find a variety of stories written by different authors with different interpretations of their relationship.
Survival scenarios are also quite common if these characters are from a post - apocalyptic or action - oriented anime. They might have to rely on each other in dangerous situations, which can bring out different aspects of their relationships. For example, Takashi might have to protect Rei and Saeko from threats, and this can strengthen or test their bond.
I'm not sure which specific rei x takashi fanfiction is the best. But often, on platforms like FanFiction.net, you can find different genres. It could be a love - story type of fanfiction where their relationship develops in a sweet and heartwarming way. You should look for stories with high ratings and positive reviews.
Some dedicated anime fanfiction communities could also be a good place to look. These communities often have users sharing their own works. You can join forums related to the anime that these characters are from and ask around for Takashi x Rei x Saeko fanfiction.
You can try searching on fanfiction websites like Archive of Our Own (AO3). It has a large collection of fanfictions, and you might be able to find some great takashi x rei ones there. Just use the search function and type in 'takashi x rei' to see the available stories.
Another theme is the growth of their relationship. It might start as a simple friendship or a reluctant alliance, but as they face challenges together, it evolves into something deeper. There could be moments of jealousy or misunderstandings as well, which are common in relationships.