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.
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.
I think Tyson, Percy's cyclops half - brother, would be a good character. His strength and loyalty would be assets in any situation. Also, Nico di Angelo could be in the fanfic. His control over the dead could provide an interesting dynamic, especially if they are exploring some dark or ancient places related to Hephaestus' past works. And of course, Clarisse La Rue. Her combat skills and fierce personality would add some drama and action to the story.
Annabeth could be a great addition. She could bring her strategic mind to help Percy deal with Medusa. For instance, she might come up with a plan to use Medusa's own pride against her.
Paul Blofis should be in it. Since he's Sally's husband, his presence would add an interesting layer to the family dynamic in the fanfic. He could be involved in some humorous situations when it comes to understanding Percy's demigod world.
The main characters are obviously Percy Jackson and Piper. Percy is known for his heroic deeds and his ability to control water. Piper has her own charm - speaking powers which are very important in the story.
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.
The other demigods from Camp Half - Blood should be included. People like Clarisse, who at first might seem grumpy about the whole wedding thing but then has a change of heart and gives a really nice gift. And Nico, who could use his powers to create some amazing lighting effects during the evening reception. Their presence would add a lot of flavor to the story.
Percy is a must - have, of course. But I think Tyson could be really interesting. His strength and his unique appearance would make for some funny and exciting moments during the drive. And Nico di Angelo could also be a great addition. His powers over the dead could create some spooky yet cool scenarios, like when they pass through an old cemetery during the drive and he has to deal with restless spirits.
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.
Nico di Angelo would also be an interesting character to bring in. His dark past and his connection to the underworld could provide a contrast to the more positive and light - hearted relationship between Percy and Rachel. He might have his own motives that either help or hinder Percy and Rachel's mission.