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.
In 'Akuma Manhwa', the plot is centered around a world where magic and demons exist. The protagonist must navigate through various obstacles and battles to protect what they hold dear. It's a story filled with excitement and unexpected twists.
Harry Potter himself is a likely candidate. Given his heroic nature and the need to constantly fight against dark forces, an Akuma no Mi could be a great addition to his arsenal of skills. It would give him new ways to protect his friends and defeat his enemies.