Breathing Styles are obviously the go-to, but I'm always more intrigued by OCs who don't just copy Tanjiro's path. Sure, Sun Breathing derivatives are common, but I've seen some wicked original concepts like 'Ash Breathing'—mixes Water's fluidity with a scorched, after-burn effect. Even a simple 'Mist Breathing' OC, if written with a focus on disorientation and psychological tricks over raw power, can feel fresh.
What really makes an OC's power stand out, for me, is a fatal flaw. Like a technique that drains their lifespan with each use, or a Blood Demon Art that requires consuming their own memories. It adds stakes beyond just 'getting stronger.' Saw one fic where the OC's 'Echo Breathing' let them replay and counter any move they'd seen, but only three times a day. That limitation created way more interesting tension than another generic 'master of all elements' type.
Honestly, the most popular ones I see are still the elemental combos: Thunder and Water, Flower and Insect. They're popular because they work, offering clear visual metaphors and combat roles. But I'm secretly hoping for more weird sensory stuff, like a 'Silence Breathing' that negates sound to create openings.