One Piece, Katekyo Hitman Reborn! Recommendations
Although the theme of Reborn! and One Piece are entirely different, the values they represent are quite similar. For instance, you have the whole concept of "family" in Reborn! which complimenets One Piece's notion of a "nakama". Another similarity is that you have various organisations - different mafia groups in Reborn! as oppose to different pirate groups in One Piece. In addition, each character, in both mangas, tends to have a specific way of fighting. Thus, highlighting another similarity found between Reborn! and One Piece.
The main focus of both series is 'friendship' and the main character's of each manga will do anything possible to rescue there crew/family respectively. Both are also filled with little gags in between the action.
Both series involve the bonds of friendship, with One Piece involving the close relationships between the Strawhat crew while Katekyo Hitman Reborn! involving the close relationships between the Vongola 10. Within those both groups, the protagonists' friends are extremely loyal to their leaders.
Both main characters (Luffy and Tsuna) are extremely similar as they have the ambition to protect their friends.
Both have a vast amount of characters that have interesting and unique backgrounds.
Both series have a large amount of action and use of supernatural powers.