Katekyo Hitman Reborn!, Shaman King Recommendations
Both series have main characters who accumulate friends and allies on the basis of their unique categorization (shamans in Shaman King, mafia members in Reborn!). They also have, after a certain point, battle after battle and the accompanying need to improve their abilities to accomodate their opponents' abilities. Reborn! doesn't have Shaman King's tournament structure, but the successive showdowns and increasing danger of their enemies mimics it anyway.