Not sure I've seen as many fics for this ship as some others in the 'Kakegurui' fandom, but the ones that do exist seem to lean hard on the power imbalance dynamic. Mary is all about calculated risk and regaining control after her family's downfall, while Yumeko is pure chaos and instinct. A lot of writers pit Mary's rigid logic against Yumeko's intuitive madness, framing their interactions as a constant, high-stakes game even outside the gambling halls. It's less about romance and more about psychological warfare with romantic tension.
You'll find a ton of 'mutual obsession' tropes. Mary becomes fixated on understanding Yumeko, on predicting her, which mirrors Yumeko's own obsession with the thrill Mary provides. It creates this feedback loop where they're the only two people who can truly match each other's intensity. The 'enemies to lovers' arc is almost a given, starting from outright hostility in Season 1 and slowly morphing into a grudging, then desperate, partnership.
A niche trope I enjoy, when done well, is 'caretaker Mary'. It's a flip on the usual dynamic. Instead of Yumeko being the untouchable force of nature, you'll see fics where her gambling mania leads to a crash—exhaustion, injury, self-destruction—and a furious, practical Mary has to step in and forcibly look after her. It highlights a protective side to Mary's anger that canon only hints at.
Less common but interesting are AUs that transplant them into different settings, like corporate rivals or fantasy realms, but always keeping that core of 'order versus chaos'. The gambling is just the medium; the real conflict is their fundamental approaches to life. I kind of prefer fics that remember Mary's sharp edges aren't just for show; she's not a soft love interest waiting to happen, and reducing her to that misses the point of her character.