Yes, there might be. Fanfiction is a vast world, and with the popularity of Percy and the Greek mythology concept, it's quite possible that someone has written a story where Percy is Zeus' mate. You can check popular fanfiction websites like Archive of Our Own or FanFiction.net.
Well, it could involve Percy having to deal with the power dynamics of being Zeus' mate. Maybe there are political intrigues in Olympus that Percy has to navigate. For example, other gods might be jealous or have their own agendas regarding this new relationship.
The theme of growth is quite common in these fanfictions. Both Percy and Scott grow as characters as a result of their relationship. They become more confident, more understanding. And the theme of unity is also there. They are often portrayed as a strong duo that can overcome anything when they are united, whether it's fighting against a common enemy or solving a complex mystery.
One common theme is the discovery of their mate bond. It's often about how they initially realize they are mates. Another theme is loyalty. They have to be loyal to each other in the face of various challenges.
Since I don't have access to the 'apollos heart novel', it's difficult to determine the exact main theme. However, many novels focus on relationships, either romantic, family - related or friendships. So it might be centered around one of those aspects in relation to the character Apollo.
One common theme is loyalty. Both Percy and the werewolf characters often show loyalty to their friends and mates. Another theme is the struggle between different worlds. The werewolves come from their own world, and Percy from his demigod world, and there's often a struggle to co - exist.
The main characters are Harry and Percy. Harry, with his Gryffindor courage and magical skills, and Percy, perhaps with his own distinct powers and personality. In this fanfiction, their relationship as mates is the core element, and other characters may be introduced to either help or hinder their bond - forming process.
Well, it could be about Harry and Percy going on adventures together, facing magical creatures side by side. Maybe they start as rivals in some magic competition but gradually become mates as they realize they have a common goal, like protecting their magical world from a new dark force.
It could be the Titans who escaped from Tartarus and are seeking revenge. They might see Percy and Apollo's relationship as a threat to their plans of overthrowing the Olympians. Another possibility is some jealous gods or goddesses who don't like the idea of Percy and Apollo being mates.