I completely get the urge to find a fresh twist on 'Hunter x Hunter' pairings—the classic dynamic between Killua and Gon is so rich, but throwing them into a crossover can reveal new facets of their characters. Your best bet for these unique mash-ups is a two-pronged approach. First, Archive of Our Own has incredibly robust filtering; you can navigate to the 'Hunter x Hunter' fandom, filter for Gon Freecss/Killua Zoldyck, and then use the 'Additional Tags' field to search for 'Crossover.' Tag wranglers there do a fantastic job, so you might find them crossing over with everything from 'Jujutsu Kaisen' to 'Naruto.' The second, more old-school option is FanFiction.Net; their crossover category is huge, and you can specifically select 'Hunter x Hunter' as the base fandom and then browse. The quality can vary more there, but the sheer volume of 'transplant them into the world of My Hero Academia' stories is significant. Don't overlook Tumblr or even specific Discord servers dedicated to 'HxH' shipping, either; authors often post snippets or links to their more experimental cross-platform works there.
I stumbled on a fantastic story once where Gon and Killua were reimagined as students at Hogwarts, with their nen abilities interacting with magic in such a clever way. That's the real appeal—seeing how their unwavering loyalty and contrasting personalities translate into a completely different rule set or conflict. Finding these can feel like a treasure hunt, so when you use the search functions, try pairing the ship tag with very specific fandom names rather than just the general 'Crossover' tag. Also, some authors create 'fusion' AUs rather than strict crossovers, which might be tagged differently but scratch the same itch of seeing them in a novel scenario. The community around this pairing is wonderfully creative, and with a bit of patient digging, you'll uncover narratives that put them in video game worlds, post-apocalyptic settings, or even slice-of-life universes that highlight their domestic side in a new context.