Hunter x Hunter
Both are adventure-first shounen with massive worlds, strange powers, and arcs built around discovering how each new society works. Hunter x Hunter is more tactical and compact, but fans of One Piece's worldbuilding usually appreciate its depth.
Fairy Tail
Fairy Tail gets recommended because it has the same emphasis on found family, guild/crew loyalty, comedy, and big emotional battles. It is more straightforward fantasy and less mystery-driven than One Piece, but the friendship-and-adventure vibe is very close.
Naruto
Naruto shares the long journey of a young protagonist chasing a huge dream while gathering bonds, rivals, and painful backstories. It is more village/training focused than One Piece, but the emotional shounen payoff is similar.
Dragon Ball
Dragon Ball is the older adventure-battle template that One Piece fans often get pointed toward: goofy exploration, escalating enemies, iconic transformations, and a hero who loves freedom and fighting. It is simpler, but the adventurous spirit connects strongly.
Toriko
Toriko is a common rec because it has wild worldbuilding, bizarre creatures, huge adventure locations, and a food-driven sense of discovery. It is more battle-food fantasy than pirate saga, but the scale and weirdness feel very One Piece-adjacent.