One common theme is adventure. Since both characters are from the Demon Slayer universe, they often go on demon - slaying adventures together in the fanfictions. They might face powerful demons and rely on each other's skills to survive.
One idea could be a rengoku fanfic where he trains a new group of young slayers. It could focus on his unique teaching methods and how he instills his values in them.
Tanjiro would be a great character to pair with rengoku. Their different personalities could create an interesting interaction. Tanjiro's kind - hearted nature and rengoku's fiery determination could lead to some great storylines.