One idea could be that Naruto, with his unique charm and ninja skills, attracts a group of female characters from different villages. For example, Hinata could be part of the harem, still being shy but deeply in love with Naruto. Sakura might also be there, having finally realized Naruto's true worth. And then add some new female characters from other lands, each with their own special abilities and personalities, and they all come together because of Naruto's heroic deeds and kind heart.
Another idea is that after Naruto becomes Hokage, his fame and influence spread far and wide. Female ninjas from all over come to Konoha to learn from him or seek his alliance. In the process, they fall for his leadership, his determination, and his never - give - up attitude. Characters like Tenten could be part of the harem, along with some new international ninja girls.