A possible plot could be a love triangle. Maybe a new character enters their world and has feelings for Naru. Hiro then has to fight for Naru's love and prove that their connection is stronger. This would add drama to the story and show the depth of their relationship.
A fanfiction could focus on a parallel universe where Naru and Hiro are not Prism Stars yet. They meet in a different context, like at a music concert. They discover their shared love for Prism Shows and start their journey together from scratch. They have to overcome the odds, like getting accepted into a Prism School, and along the way, their relationship blossoms into something more than just friendship.
One possible plotline is that the thief steals something important from Hiro at first, but then falls in love during the process. The love letter could be an apology and a confession at the same time. Another could be that they are childhood friends, but the thief turns to a life of crime. The letter is a way to come back to Hiro.
Some plots may revolve around self - discovery. Rainbow Dash and Rainbow Blitz could be turned into humans and have to figure out who they are without their pony identities. They might learn about human emotions and relationships in a deeper way. There could also be a plot where they are training humans in the ways of the ponies, like teaching them about loyalty and friendship. Or they could be on a quest to find a magical item in the human world that has some connection to their pony world, facing many challenges and making new friends along the way.
One possible plot could be Garble trying to prove himself to Rainbow Dash. Maybe he starts off as his usual rough self but gradually changes as he gets to know her better. He could be involved in some sort of flying competition where Rainbow Dash initially underestimates him.
One possible plot is that Rainbow Dash is caught in a battle between dark magic and the forces of good. She might be overpowered by the dark magic and die as a result. This would be a very dramatic plot as it would involve the struggle between good and evil in the My Little Pony universe.
One common theme could be forbidden love, as there might be elements in the original story that make their relationship not straightforward.
A plot where Naru gets sick and Keitaro takes care of her. He makes her soup, reads her stories, and just generally shows how much he loves her. It's simple but very heartwarming.
A possible plot could be that Sinbad and Proteus are both vying for the same position in their kingdom. This creates a lot of tension between them as they both have different ways of approaching leadership. However, as the story progresses, they realize that they can achieve more by working together.
Brotherly love is a very common theme. Since they are brothers in the original story, fanfictions often expand on their deep bond, like how they protect each other. Another theme is mentorship, as Tadashi was like a mentor to Hiro in some ways.
The very idea of 'tadashi x hiro lemon fanfiction' is quite inappropriate as their relationship in the original context is that of brothers. If such fanfictions exist, they would be a misrepresentation of their relationship. They might have elements that are not in keeping with the positive, family - centered values of the original story, like inappropriate physical or emotional relationships that are not part of the true nature of Tadashi and Hiro's connection.