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.
You can try websites like Archive of Our Own (AO3). It has a large collection of fanfictions from various fandoms, and there's a good chance you'll find Tamaki and Shinra love fanfiction there.
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.
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.
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.
Common themes include friendship turning into love. Maybe they start as friends who support each other in their daily lives or adventures, and gradually those feelings turn romantic. Growth is also a big theme. As they fall in love, both Shinra and Maki might grow in their magical abilities or personal development. And of course, the theme of loyalty, where they stay true to each other no matter what difficulties come their way in the fanfiction world.
One possible story could be that Shinra and Maki meet in a bustling city. Shinra, with his kind heart, helps Maki when she's in a bit of trouble. They start spending more time together, going to cafes and parks. As time passes, they realize they have deep feelings for each other. They share their first kiss under a cherry blossom tree, and from then on, their love story unfolds with many sweet moments.
One popular one might be 'A Chance Encounter' where Tamaki and Haruhi's relationship develops in a really sweet way. They have a series of accidental meetings that lead to deeper feelings.
You can try Archive of Our Own (AO3). It has a large collection of fanfiction from various fandoms, and there are surely many Tamaki and Haruhi love stories there.