Well, Wyatt and Chris, her sons from the future in the original series, could be interesting characters to include. They could come back in time to make sure their mother has a safe pregnancy. Or perhaps some of the demons that the Charmed Ones have faced in the past could make an appearance, but this time not for attacking but for some strange reason related to the pregnancy. And of course, the Elders could be involved, either to help or to create some obstacles depending on the plot.
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.
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.
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.
Chat Noir is an obvious choice. His relationship with Ladybug is so central to the original story, and with her pregnant, his reactions and support would be a great addition. Marinette's best friend Alya could also be included. She could be the one Ladybug confides in, and Alya could use her journalistic skills to help keep Ladybug's pregnancy a secret if needed. And Master Fu could be in the story. He could offer his wisdom and guidance, especially when it comes to any potential threats to Ladybug or the unborn child.
Leo Valdez would also be an excellent character to include. His mechanical skills and sense of humor could lighten the mood during tense situations. He could also create some cool gadgets to help the group in their adventures, like a device to detect mythical energy or something similar.
Mentor - like characters can also be great in a Percy Piper fanfic. They can guide Percy, pass on wisdom, and help him grow as a character. Maybe an old wizard who has been around for ages and has a lot of knowledge about the magical world.
Polly could be a great supporting role. Since she has a history with Piper, she could bring in some family - related drama. Also, Red would be an amazing addition. Her motherly nature towards Piper in prison could translate into her being a sort of mentor or a source of advice in their married life.
Definitely their friends from the show. Bonnie could be a great addition as she might be able to use her magic to protect Caroline during the pregnancy. Klaus could also be an interesting character to include. He has a complex relationship with both Tyler and Caroline, and his presence could add some drama and conflict, like he might try to use the baby for his own purposes.
Finn Hudson could be an interesting character to include. He might be a bit clueless at first but then grow to be very protective of Emma and the baby. Santana could add some drama and spice to the story. Her sharp tongue could either be used to support Emma or cause some trouble, depending on the plot. Puck could also be in the story, perhaps he offers a different perspective on fatherhood since he has his own experiences related to it in the show.
Brittany's parents should be in the story. Their reactions to her pregnancy could range from disappointment to full - on support, and this would add a lot of depth to the story. Also, Tina. She could be a great friend to Brittany, sharing her own fears and hopes for the future. And don't forget Finn. He could be the one who encourages Brittany to keep going with the glee club, saying that it's important for her and the baby to have that creative outlet.
Poseidon should definitely be included. His relationship with Sally and his reaction to the pregnancy would add a lot of depth. Percy Jackson, their son, could also be a great addition. His concern and protectiveness towards his mother during this time would be interesting to explore.