One idea could be the OC having to navigate through different elemental cultures within the Avatar world to gain the trust and love of the harem members. For example, if part of the harem is from the Water Tribe, the OC might have to learn their traditions and help with community issues like water shortages. Another plot could involve the OC having a unique bending ability that draws the attention of the harem. Maybe they can bend a new element or have a special way of using an existing one. This would make them both a target and a hero in the Avatar universe.