One of the most consistent ways I've seen writers handle Hisoka and Machi is by leaning heavily into the unspoken tension of the original source material. Togashi gave us that one incredible scene—the 'stitch me up' moment after Hisoka's 'death'—and fanfiction tends to expand on that precise energy. It's never just gratitude on his part or professional duty on hers. Writers latch onto the physical intimacy of the act, the weird vulnerability he shows only to her, and her clinical, almost annoyed, detachment that's clearly a front. The dynamics usually spin out from there, either as a slow-burn professional partnership that curdles into something else, or a much darker game of cat-and-mouse where Machi is the only one who isn't entirely a mouse.
Personally, I think the best fics avoid making it outright romantic in a conventional sense. Hisoka's sexuality and motivations are so deeply intertwined with his battle lust that translating that into a standard relationship feels off. The more compelling interpretations frame Machi as a fascinating object to him precisely because she resists being one. She sees his tricks, she's wary, she's not dazzled. That makes her a puzzle, and Hisoka loves puzzles more than he loves anything. I've seen a few stories that explore her as the one member of the Troupe who understands his brand of chaos because she operates on a different, more precise, wavelength. It's less about hearts and flowers and more about two dangerous people recognizing a similar, if oppositional, competence.
There's also a subset of fics that flip the script and make Machi the proactive one, which I find refreshing. Hisoka is so often the instigator, the chaotic variable. Stories where Machi decides to use him for her own ends—maybe to gain an edge within the Troupe, or to settle a personal score—add a layer of agency that her canon character sometimes lacks. It turns the dynamic from 'obsessive magician stalks stoic nen-stitcher' into a mutual, if treacherous, dance. They're both using each other, fully aware of it, and the tension comes from waiting to see who blinks first. That feels way more true to both of them than a bunch of forced confessions.