The plot could start with Rhea being a powerful entity from another realm who is drawn to the Naruto world. She gives birth to Naruto, but for some reason, she has to leave him. Naruto grows up not knowing his true mother. As he progresses in his ninja journey, he starts to notice strange things about himself that don't seem to fit the normal ninja abilities. Eventually, he discovers his connection to Rhea. This could lead to a whole new arc where he has to travel to Rhea's world to learn more about his heritage and also to stop an impending threat that is related to his birth and Rhea's past.
Maybe Rhea is in hiding in the Naruto universe and falls in love with a shinobi. Naruto is born from this union. He might be ostracized at first because of his different origin. But as he discovers his mother's true identity and powers, he uses them to bring about a new era of peace between different factions in the ninja world. He could also have to face off against enemies who want to use Rhea's power that is now within him for their own evil purposes.
Well, one possible plot could be that Rhea somehow ends up in the Naruto world and has a child with a character there, who could be Naruto's father figure. Naruto then grows up with some unique powers or abilities passed down from Rhea, and he has to deal with the reactions of the ninja world to his strange heritage.