Man, you're asking about a crossover sweet spot that's honestly tricky. Gon and Killua's dynamic is so specific—that intense, childhood-friends-to-found-family bond with all the unspoken loyalty—it doesn't always mesh cleanly with other universes. When it works, though, it's magic. A fic I keep coming back to is 'Transmigration of the Unyielding' on AO3, which drops the boys into the chaos of 'Jujutsu Kaisen'. The author nails how Gon's sheer stubbornness would interact with cursed energy and how Killua would instantly clock Gojo as a threat. It's less about the big battles and more about them navigating a world just as morally grey as theirs, but without each other as an initial anchor. Really explores that 'us against the world' vibe.
Your best bet is definitely Archive of Our Own. The tagging system is your friend here. I'd search the 'Hunter x Hunter' fandom tag, then filter for Crossovers and sort by kudos or bookmarks. Sometimes you gotta get creative with the second fandom filter—try shonen series with similar themes of found family and high stakes, like 'My Hero Academia', 'Naruto', or 'Demon Slayer'. I've seen some interesting ones with 'The Promised Neverland' too, though those lean way darker. FanFiction.net has a dedicated crossover section for 'Hunter x Hunter', but the quality and tagging are way more hit-or-miss; it feels like digging in an attic. Sometimes you strike gold with a decade-old fic that has a wild premise, like them ending up in 'One Piece'. The character voices can be shaky, but the adventure spirit fits.