One idea could be that Naruto somehow gets transported to the Avatar world and his unique ninja skills draw the attention of multiple female characters. They might be intrigued by his determination and power, leading to a harem - like situation where they all vie for his attention.
In a 'naruto x harem avatar fanfiction', Naruto could be on a quest in the Avatar world to find a way back home. Along the way, he meets different female characters who offer to help him. As they travel together, they develop feelings for him. One girl might be a powerful earth - bender who admires Naruto's never - give - up attitude, another could be an air - nomad who is fascinated by Naruto's spiritual side which is related to his own journey in the Naruto world, and so on, gradually forming a harem around him.
Perhaps Naruto's chakra nature has some sort of connection to the bending in the Avatar world. This could make him a sort of anomaly that the female characters, who are benders or otherwise important in that world, are curious about. They start to form relationships with him, creating a harem scenario as they each try to understand his power and where he comes from. For example, a water - bending female might see similarities between his chakra control and her bending, and be drawn to him.
Another plot could be that Naruto is mistaken for an Avatar in the Avatar world. As he tries to clear up the misunderstanding, he gets involved in local politics and relationships. The female characters see his honesty and kindness, and gradually, a harem - like situation develops around him.
In a naruto x avatar fanfiction, there could be a scenario where the Akatsuki from Naruto's world teams up with some of the villains from the Avatar world. This would force Naruto and the Avatar (whichever one it may be) to put aside their differences and form an alliance with other characters from both worlds to stop this powerful and evil coalition.
One idea could be that the OC is a powerful bender who saves a group of characters of different genders from a great danger. In return, they all develop feelings for the OC and form a harem. For example, if the OC is a water bender, they might save some earth benders and fire benders from a group of bandits. Another idea is that the OC has a special ability that is needed to save the Avatar world, and as they travel around recruiting help, different characters join them and fall in love.
One idea could be Naruto somehow finds himself in a situation where he has to team up with Tayuya frequently. As they interact more, a harem starts to form around Naruto, with Tayuya being one of the key members. The plot could involve them going on missions together and Naruto gradually realizing his feelings for Tayuya among the others in the harem.
One idea could be that the characters from Avatar: Pandora discover a portal to the Naruto universe. They are amazed by the ninja abilities and chakra system. Some Pandora characters might try to learn ninja techniques, while Naruto characters are intrigued by the unique flora and fauna of Pandora.
One idea could be that Naruto accidentally enters a different dimension where he meets Fuu and gradually attracts other characters who form a harem around him. They could go on adventures together, fighting against new threats in that dimension while Naruto tries to find his way back home.
Maybe an evil villain casts a spell on Naruto that draws a harem of female ninjas to him, thinking it will distract him. But Naruto uses this new situation to his advantage. He has the girls help him train in new ways and they all form a unique bond as they work together to face the real threats to the village.
One idea could be that Naruto, while on a mission in an unknown land, meets Guren and gradually other characters are attracted to him, forming an unexpected harem. Maybe Naruto's unique ninja skills and his kind heart draw them in.
A more complex plot could be that there is a powerful enemy that threatens both Naruto's world and the world of Rias. Naruto and Rias's harem have to team up to defeat this enemy. Along the way, they face many challenges that test their abilities and relationships. Naruto's positive attitude and never - give - up spirit start to influence the members of the harem, and they in turn show Naruto new ways of using magic and power.
One idea could be that Naruto discovers a hidden realm full of monster girls. They are initially wary of him but as he shows his kind heart and powerful ninja skills, they start to fall for him. For example, a fox - like monster girl might be drawn to his own connection with the nine - tailed fox.