Well, perhaps a story where Percy and Artemis discover a prophecy that predicts their separation. They then strive to change their fate. This could involve them traveling through time, seeing different versions of their relationship in the past and future. Percy might use his wits to find loopholes in the prophecy, while Artemis uses her divine powers to fight against the forces that seem to be pushing them apart. They could also meet other deities who either help or hinder their quest to stay together.
A plot could involve Percy and Artemis going on a quest to find a lost artifact that has a connection to both of their pasts. This could lead to them exploring new realms and encountering old friends and foes. Along the way, they could face challenges that test their love and loyalty to each other. Also, Percy could use his skills and knowledge from his previous adventures to help Artemis in situations where she might be at a disadvantage, like in the mortal world.
One plot idea could be Percy and Artemis facing a new and powerful enemy that threatens their relationship and the world they protect. For example, a dark sorcerer who aims to steal Artemis' powers and uses Percy as a way to get to her. Another could be Percy trying to adapt to the life among Artemis' followers, dealing with their unique customs and rules. Maybe he has to prove himself worthy to be by Artemis' side in a series of trials set by the goddess herself.