Fullmetal Alchemist: Brotherhood
Both are adventure stories with a huge world, a strong emotional sibling/friendship core, and fights that rely on rules instead of raw yelling. Brotherhood is more tightly plotted, but it has the same balance of humor, tragedy, and moral complexity.
Yu Yu Hakusho
This is the natural rec because it comes from the same creator, Yoshihiro Togashi, and you can feel the shared DNA in the tournament arcs, character banter, and team dynamics. Fans of Hunter x Hunter's strategic fights usually appreciate Yu Yu Hakusho's older-school version of that style.
Naruto
Both follow young fighters growing through exams, mentors, rivalries, and increasingly dangerous arcs. Naruto is more emotionally direct, but the chunin-exam style structure and tactical ability matchups are a big reason fans cross-recommend them.
One Piece
The shared appeal is adventure: big world, long-term mysteries, weird powers, and arcs that build entire societies before tearing them apart. One Piece is more comedic and sprawling, but fans who loved Hunter x Hunter's worldbuilding usually understand why this gets recommended.
JoJo's Bizarre Adventure
Both shows are loved for power systems where creativity matters more than simple strength. JoJo is louder and more flamboyant, but the strategic 'how do we beat this specific ability?' structure lines up well with Nen battles.