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.
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.
You can try fanfiction platforms like Archive of Our Own (AO3) or FanFiction.net. There are often many talented writers who create stories related to 'Hitman Reborn' with original characters (OCs) that are thieves. Just search for the relevant tags on these platforms.