One possible fanfiction could be where Tsuna and Chrome go on an adventure together to find a special item in the Katekyo Hitman Reborn world. They might face various challenges along the way and their relationship could grow stronger as they rely on each other's skills.
There could be a fanfiction where the 'game on' concept is taken to an extreme. All the characters are forced into a real - life - or - death game, and they have to use their skills and powers in ways they never thought of before. This would create intense and thrilling plotlines, with characters like Reborn guiding Tsuna through these dangerous challenges.
Create an engaging plot. Maybe start with a unique 'game on' concept, like a virtual reality game that the characters are sucked into. Then, build up the tension and conflict. Have the characters face challenges that test their limits, like fighting against their own doppelgangers in the game world. And don't forget to add some twists and turns to keep the readers hooked.
A plot could be that Tsuna and Chrome are accidentally sent to an alternate dimension. In this new place, they have to figure out how to get back home. Along the way, they discover new powers within themselves and their relationship evolves as they depend on one another for survival. This would be a great way to explore their characters more deeply.
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.
The unique character design of the thief OC. If the writer creates a complex and engaging character with a distinct personality, backstory, and set of skills related to thievery in the 'Hitman Reborn' universe, it can be really interesting.
One interesting crossover is 'The Hitman in Hogwarts'. It combines the unique assassination skills from 'Hitman Reborn' with the magical world of 'Harry Potter'. The story follows a character similar to Tsuna from 'Hitman Reborn' who finds himself in Hogwarts. He has to deal with the challenges of both worlds, like facing the Death Eaters while also trying to control his newfound magical powers.