The main antagonist could also be a new and unknown force from the Underworld. It could be a powerful spirit or entity that is spreading despair among the demigods. Percy tries to fight against it but finds it so difficult that he begins to lose hope in the face of this seemingly invincible foe.
One possible storyline could be Percy losing hope after a major betrayal by a close friend in the demigod world. Maybe he feels like he can't trust anyone anymore and starts to isolate himself from his usual group. This could lead to a downward spiral where he even questions his own abilities and purpose.
One consequence could be that he might isolate himself from his friends. In fanfiction, Percy is often depicted as a social and heroic figure. Losing hope could make him feel like he's a burden to others and cause him to withdraw. For example, he might stop showing up at Camp Half - Blood activities.
You can try searching on fanfiction websites like FanFiction.net or Archive of Our Own. They have a large collection of Percy Jackson fanfictions. Just use the search bar and type in relevant keywords related to the 'lost' aspect, like 'lost chapter' or 'lost story' along with 'Percy Jackson'.
His friends could play a crucial role. For instance, Annabeth could remind him of all their past victories and how they overcame seemingly impossible odds. Just like in the original series, her logical thinking and belief in Percy could start to chip away at his hopelessness.
One common theme is the clash of their different worlds. Hope comes from a world of vampires, werewolves, and witches, while Percy is from the world of Greek gods. So, how they deal with the differences in magic and power systems is often explored. Another theme is friendship or romance. Writers like to imagine how they would interact on a personal level, whether they become friends or something more.
One common theme is the mystery surrounding their relationship. Since Percy is her lost husband, there's often a search for the truth about their past together. Another theme could be the clash of different worlds, as Percy is a demigod and Artemis is a goddess. Their union might bring about conflicts within the divine and mortal realms.
There are quite a few fanfictions on this topic. You can start by checking popular fanfiction websites like Archive of Our Own. Some might explore the unique relationship dynamics between Percy and Artemis in a world where he is her lost husband. However, the quality can vary greatly, so it's best to read the reviews and summaries before diving in.
One idea could be a crossover where Percy and Hope team up to fight a common enemy that threatens both the demigod and the supernatural worlds. Maybe an ancient evil that has the power to disrupt the balance between the gods and the vampires/werewolves/etc.
One possibility could be that she shows up at Camp Half - Blood unexpectedly. Maybe she was raised by a mortal family all this time, not knowing about her demigod heritage. When she arrives, she has powers similar to Percy's but also some unique ones, like the ability to communicate with sea creatures in a different way than Percy. This could lead to all sorts of adventures and conflicts within the camp as everyone tries to figure out who she really is and what her role will be.