One great fanfic could be 'The Unlikely Bond'. It delves into how Hibari Kyoya and Tsunayoshi Sawada, who seem so different at first, gradually form a unique connection. Their interactions are filled with both tension and understanding as they face various challenges together in the story.
Another theme is self - discovery. By going back in time, Tsunayoshi gets to see himself from a different perspective. He realizes his own growth and potential, and this helps him to become a stronger person both in the past and for his future self. In some fics, he discovers new abilities or talents that he didn't know he had before.
Protection is also a prevalent theme. Hibari Kyoya is a powerful fighter, and in many fanfics, he ends up protecting Tsunayoshi Sawada in various dangerous situations. This creates a sense of loyalty and deepens their bond. Sometimes, it's not just physical protection but also emotional support that they offer to each other.
One great fanfiction could be 'Tsunayoshi's Time - Rewritten'. In this fic, Tsunayoshi is sent back and uses his future knowledge to change many things in the past. It has an interesting plot that focuses on his relationships with his friends and how they develop differently this time around.
One idea for an OC in Katekyo Hitman Reborn could be a gamer who has the ability to use video game skills in the real world of the series. For example, they might have a 'health bar' that others can see, and they can level up their combat skills like in a game. This would add an interesting dynamic to the story as they interact with the existing characters.
There could be a fanfic that focuses on a romantic relationship between a character from Katekyo Hitman Reborn and Sakura. Let's say Hibari falls for Sakura when she stumbles into his territory. He is initially hostile but then gradually becomes interested in her unique charm, and the story could explore how their relationship develops against the backdrop of the Reborn world.
The Hitman Reborn manga ended with a climactic battle and resolution of the main character's story arcs. But it's a complex ending that's best discovered by reading it yourself.