Another plot could involve Tamaki getting injured during a fight. Shinra then takes it upon himself to take care of her. As he nurses her back to health, they have a lot of time to talk and get to know each other on a more personal level. This could lead to them realizing that they have a lot more in common than they initially thought, and their relationship could grow from there.
In a fanfic, their relationship could also develop through moments of vulnerability. Let's say Shinra has a nightmare about his past and Tamaki is there to comfort him. Or Tamaki has a moment of self - doubt about her abilities, and Shinra gives her an encouraging speech. These moments of showing their true selves and being there for each other can deepen their connection. It's about creating situations where they can be honest with each other and build an emotional bond.
The OC could be a scientist who is working on a new way to control the Infernals without having to kill them. Tamaki is assigned to protect the OC as there are those who oppose this new method. As they spend time together in the lab and during field tests, Tamaki becomes fascinated by the OC's intelligence and dedication. The OC, in turn, is charmed by Tamaki's positive attitude. They might face opposition from within the Fire Force, but their relationship and the OC's research continue to progress.
One idea could be that Naruto's ability to control chakra is somehow combined with the Fire Force's pyrokinetic abilities. For example, Naruto might teach the Fire Force members some of his chakra control techniques, which could enhance their ability to control their flames in a more precise way.
One idea could be that Ghost Rider somehow gets transported to the world of Fire Force. He could team up with the Fire Force members to fight against a new type of infernal that has powers related to the underworld. His Penance Stare might be used to expose the true nature of some of the more mysterious infernals or even some corrupt individuals within the Fire Force organization.
A sense of loyalty is often a theme. In these fanfics, Shinra and Tamaki are shown to be loyal to each other. Whether it's facing external threats or internal doubts in their relationship, they stand by one another. And the theme of shared dreams can also be found, like they might have a common goal for the future.
One common trope could be 'opposites attract'. Tamaki might be outgoing while Shinra is more reserved, and this contrast can create an interesting dynamic in a love story. Another could be 'friends - to - lovers'. They start as friends, gradually realizing their deeper feelings for each other.
I like the fanfic 'Shinra and Tamaki: A New Beginning'. In this one, it starts with a new situation for both of them. It explores their feelings in a fresh way, like how Shinra first realizes his deeper feelings for Tamaki and how Tamaki responds to his newfound affection.
Another way is through their relationships. In the case of a Tamaki x OC fanfiction, their interactions with Tamaki are crucial. If the OC is initially shy around Tamaki due to her confident and outgoing nature, but gradually opens up, it shows character growth. Also, their relationships with other members of the Fire Force can contribute. Maybe they have a rivalry with one member that forces them to improve their skills, which in turn affects their overall character development.
You could write a fanfiction that delves deeper into the relationship between Arthur and Shinra. Maybe they get into a big argument over their different approaches to being a hero. Shinra believes in using his powers for the greater good in a more practical way, while Arthur is all about living out his 'knight in shining armor' fantasy. This argument could lead to them going their separate ways for a while, and then having to come back together to face a powerful Infernal that neither of them could defeat alone.
Well, you could create an OC fanfiction where the OC is a scientist who discovers a new way to combat Infernals that doesn't involve burning them. This OC might be initially rejected by the Fire Force but then proves their method's worth. Also, an OC with a special connection to the Adolla Burst could be interesting. Maybe they can communicate with the spirits of the Infernals or have visions related to the Adolla realm, which would lead to new adventures and revelations within the Fire Force world.