One possible storyline could be Naruto finding himself in the Charmed Ones' world and teaming up with Piper to fight demons. Naruto's ninja skills would be a great asset against the magical threats they face.
One popular storyline could be Piper dealing with the challenges of her pregnancy while still protecting the world from evil. She might have to balance her new physical limitations with her duties as a Charmed One. Another could be how her sisters react and support her during this time. Maybe they use their powers in unique ways to help Piper through the pregnancy.
One idea could be a crossover where Piper uses her magic to visit Hogwarts. She could interact with the students and teachers, perhaps teaching them some unique spells from her own magical world.
For Naruto and Piper, a great character to pair them with could be Sasuke. Sasuke's brooding nature and powerful skills would add an interesting dynamic. He could also learn a thing or two from Piper's magic.
There might be a fanfiction where Piper gets transported to the Naruto universe and she teams up with Naruto to fight against a common enemy. She could use her skills with the dagger and her magical powers in combination with Naruto's ninja techniques to defeat powerful foes.
Harry Potter himself could be a great focus. Imagine him being introduced to the Halliwell sisters' way of life. His sense of justice and determination would be put to the test in a different magical context, and his relationship with Piper could develop in interesting ways as they learn from each other's magical experiences.
One interesting storyline could be Piper trying to understand Dark Ace's motives. Maybe she discovers that he has a past trauma that led him to be on the 'evil' side. This could create a complex relationship where Piper wants to help him change.
One interesting Naruto Pied Piper fanfiction could be about the Pied Piper using his powers in the Naruto universe to control some of the summons. For example, he might be able to make the toads or snakes from the summonings act in ways that no one expected. This could create a whole new set of adventures for Naruto and his friends as they try to figure out what's going on and how to stop it.
Definitely her sisters, Phoebe and Paige. They are her closest family and would be there for her throughout the pregnancy. Leo, as her husband, is also crucial. He can provide support and love. Then there could be some of the recurring good witches or magical beings who could offer their help or advice. For example, Grams could appear in Piper's dreams to give her some grandmotherly wisdom about motherhood.
Another storyline might focus on the relationships between the Sand ninja and the Leaf ninja. For example, how they overcome their past differences and work together on a mission. It could involve a lot of character development and understanding between different cultures in the Naruto universe.
One interesting storyline could be a crossover where Naruto somehow ends up in Hippolyta's world. He might use his ninja skills to help the Amazons in their battles, like using his shadow clone jutsu to create distractions during an enemy attack.