Not gonna lie, I tend to steer clear of the super fluffy domestic stuff for these two. It feels off. The appeal of Leorio and Gon, for me, is the adult-young person mentorship that's got this weird, unspoken charge to it. Like, Leorio is trying so hard to be the responsible one, but Gon's chaotic innocence just flips his script completely.
I'm a sucker for the 'found family' tag, but specifically the angsty versions. Stories where Gon's self-destructive tendencies post-Chimera Ant arc really come to the forefront, and Leorio has to wrestle with his medical ethics versus his sheer fury at Gon for not valuing himself. There's a tension there that's way more interesting than just them holding hands. Hurt/comfort works, but only if the 'comfort' part is messy and argumentative.
Also, a weirdly specific niche I search for: canon-divergence where Gon never loses his Nen. That power imbalance shifts everything. How does Leorio, now the clearly less 'powerful' one in a Hunter sense, assert his protective role? It creates a different friction that a good writer can really mine for gold.