One common theme is probably the development of a romantic relationship. Writers often explore how the reader character and Amemiya Taiyou fall in love, from the initial attraction to a deeper emotional connection. Another theme could be adventure, where they go on some sort of journey together, either in a fantasy world or within the context of the original story's setting.
You can try searching on popular fanfiction websites like Archive of Our Own (AO3). It has a large collection of fanfics across various fandoms, and you're likely to find some great Amemiya Taiyou x Reader fanfictions there. Another option is Fanfiction.net, which also has a wide range of works.
One popular type of inazuma eleven go taiyou x reader fanfiction story might be those where the reader character joins the team. For example, they could be a new player with a unique skill that helps the team in crucial matches. Another could be a story where the reader character has a special bond with Taiyou, like a friendship that develops over shared interests in soccer. There are also fanfictions where the reader character is from a different world and gets involved with Taiyou and the team's adventures in the Inazuma Eleven Go universe.
One common theme is adventure. Since Shiro is often associated with heroic deeds and battles, many fics will have the reader joining him on some kind of intergalactic adventure. Another theme is trust. As Shiro is a leader, the relationship between him and the reader might develop as they learn to trust each other in difficult situations. There's also the theme of healing. Shiro has been through a lot, and some fics will explore how the reader helps him heal from his past traumas, and vice versa.
One common theme is the relationship development between Karma and the reader. It could start from a simple acquaintance and gradually grow into a deep friendship or something more. Another theme is adventure. They might go on some exciting adventures together, facing challenges and solving problems side by side.
Another theme could be the exploration of Stein's scientific side in relation to the reader. For example, the reader might be involved in Stein's experiments in some fanfictions, leading to a deepening of their connection. Also, themes of trust and acceptance are quite common, as Stein is a complex character and the reader has to learn to accept him as he is.
There are also fanfictions that focus on the power dynamics in the relationship. Hordak is a powerful figure in the She - Ra world, and how this power affects his relationship with the reader can be an interesting theme. Some stories might explore how the reader challenges his power, while others might show how they are influenced by it. And sometimes, the theme of acceptance - of Hordak's past, his differences, etc. - by the reader is also explored.
One common theme is friendship - to - lovers. It often starts with the reader and Patton being friends and then developing deeper feelings. Another theme could be comfort. Patton is usually portrayed as a very kind and comforting character, so fanfictions might focus on him comforting the reader during a difficult time. There's also the adventure theme, where the reader and Patton go on some sort of adventure together.
One common theme is adventure. Phineas is often depicted going on wild adventures with the reader. Another theme is friendship turning into love, showing the development of their relationship.
A common theme is the 'bad boy' allure of Gaston. The reader is often drawn to his confidence and physical strength. Another theme is redemption. Sometimes Gaston is portrayed as having a chance at changing his ways because of his relationship with the reader.
One common theme is the exploration of Blitzwing's different personalities. Since he has three, it can make for an interesting dynamic in the relationship with the reader. Another is the idea of the reader being in a dangerous or exciting situation in the Transformers universe and Blitzwing coming to their rescue.