I've seen a lot of OCs try to force themselves into the Hashira ranks or be another secret sibling of a main character, and honestly, it often feels clunky. The world of 'Demon Slayer' is surprisingly vast—think about the Kakushi, the support corps. They're constantly in the background, managing logistics, tending to wounded slayers, and cleaning up after battles. An OC as a skilled Kakushi medic or a blacksmith's apprentice who helps repair and modify Nichirin blades could slot in perfectly. They'd have organic reasons to interact with the main cast during recovery periods at Butterfly Mansion or supply runs, without needing to overshadow the established power scaling.
You could also explore a character from a region barely touched in the manga, like someone defending a remote mountain village with unique local demon lore. Their integration point could be during a mission where Tanjiro's squad is passing through, offering help or crucial local knowledge. The key is making the world feel bigger, not inserting a new centerpiece.
My own OC started as a Tsuguko for a minor, retired Water Hashira, which gave her a reason to be trained but not overwhelmingly powerful. She crossed paths with the main group during the Mugen Train aftermath, helping with evacuation. It felt more grounded than her suddenly showing up at the Hashira meeting.