One idea could be that the male OC is reborn into the Uzumaki clan during a time of great turmoil. He could use his knowledge from his past life to help the clan survive and thrive. Maybe he knows about upcoming threats and can prepare the Uzumaki for them.
Another plot could involve the male OC being reborn as an Uzumaki and having to deal with the discrimination the clan faces. He could work towards changing the perception of the Uzumaki within the ninja world by showing off their unique abilities and strong will. He might start by impressing the local villagers with his Uzumaki powers like strong life force and sealing jutsu skills.
One idea could be an Uzumaki male OC who discovers a hidden Uzumaki jutsu that was lost over time. He has to master it while also facing the challenges of being an outsider in the village as many are suspicious of his new power. Another plot could involve the OC having to form an alliance with the Akatsuki to save his family, but he has to keep his true motives hidden from them. Or perhaps the OC is on a quest to find out the real reason behind the destruction of the Uzumaki clan and uncovers some dark secrets along the way.
A great plot could be that the OC Uzumaki is a healer. Naruto gets seriously injured in a battle, and she uses her Uzumaki healing abilities to save him. As a result, they become closer. Then, they find out that there is a conspiracy to eliminate all Uzumaki descendants. So they work together, using their combined skills and the power of the Uzumaki heritage to fight against this conspiracy.
He might also have the power to sense chakra more acutely. This would help him in detecting enemies from a distance, finding hidden traps that are chakra - based, and also in understanding the flow of chakra within his own body better. He could use this power to train more effectively and also to teach others about chakra control.
One plot idea could be that the male OC is reborn as a marine. But he realizes the corruption within the marines and decides to secretly help the pirates who are actually the good guys in certain situations. Another idea is that he's reborn near the Grand Line and has to quickly learn the ways of this dangerous world. He could discover an ancient prophecy that involves him and the One Piece itself. Or perhaps he's reborn with the ability to understand the language of the sea kings, which gives him an advantage but also makes him a target.
One idea could be that the tall OC has a special ability related to their height, like being able to reach high places easily to retrieve important items in the world of Khr Reborn. This could lead to them being recruited by a certain group.
One possible title could be 'Naruto Uzumaki and the Unforeseen Love with OC'.
Well, perhaps the OC reborn as Kurama tries to find a way to coexist peacefully with the jinchuriki in a different way than in the original story. They could use their unique perspective to broker better relationships between the tailed beasts and the humans. Maybe they also try to uncover secrets about the origin of the tailed beasts that were previously unknown.
One idea could be an OC who has a special power related to time manipulation in the 'hitman reborn' universe. They could use this power to change the outcome of battles or prevent certain tragedies. Another could be an OC that is a long - lost relative of one of the main characters and has a different view on the mafia world, causing some internal conflicts.
One idea could be that the OC, reborn as Naruto, uses their prior knowledge to quickly master jutsu. For example, they might know about the Rasengan from the start and be able to perfect it faster. Another plot could involve the OC changing the relationships in the village. Maybe they befriend Sasuke earlier and prevent his defection. Or they could introduce new ninja techniques from their past life to the Hidden Leaf.
Another plot could involve an OC reborn into the body of a criminal in the Naruto world. They have to struggle to redeem themselves and prove that they are not the same person as the original criminal. Along the way, they could discover the real reasons behind the original criminal's actions and try to right the wrongs that were committed.