Well, it could be that Percy is fighting Ares to protect his friends. Maybe Ares has threatened Percy's loved ones in Camp Half - Blood, so Percy has no choice but to face the god of war.
In this fanfiction, perhaps Percy is fighting Ares because of some sort of power struggle. Ares might be trying to take over Percy's domain or something related to his powers. Percy, being the hero he is, stands up to him. He uses his skills with water and his lightning - fast reflexes to dodge Ares' attacks and strike back with his sword. The fight could take place in a mythical arena, with gods and demigods watching from the sidelines, all holding their breaths as Percy battles the powerful and fierce Ares.
One possible plot is that Percy is manipulated into fighting Ares. There could be a third party, like a new evil force, that tricks both Percy and Ares into fighting each other. Percy initially thinks Ares is the bad guy in this situation but later realizes they've been played. However, by that time, they're already in the middle of an intense battle, and they have to figure out how to stop fighting and join forces against the real enemy.
Well, perhaps Percy has lost his original family or home in this fanfic. Ares, being the god of war, might see potential in Percy. He could adopt Percy to train him to be a great warrior. Percy might struggle with Ares' rough and war - like ways at first, but then gradually adapt and learn unique combat skills from him.
It's hard to say. Percy has his wits and his special powers like controlling water. But Ares is a god, very powerful. If Percy can use his environment well, like creating a big wave to distract Ares, he might have a chance to win.
Well, it could be about Percy discovering he has this unexpected connection to Ares. And then he has to deal with the reactions of his friends like Annabeth. Some of them might be distrustful of this new relation. Percy would then try to find out more about his connection to Ares through exploring old prophecies and ancient sites, all while trying to keep his regular demigod duties in check.
One possible plot could be that Ares and Aphrodite's secret relationship is somehow discovered by Percy and his friends. Percy then has to decide whether to expose them or use this knowledge to his advantage in the ongoing power struggles among the gods. Maybe Aphrodite tries to charm Percy into keeping the secret, while Ares threatens him. This would create a lot of tension and moral dilemmas for Percy.
There could be Clarisse. Since she is Ares' daughter in the original Percy Jackson series, she might play an important role. She could be either jealous of Percy getting her father's attention or become a sort of mentor - like figure to him. Another character could be Annabeth. She might be worried about Percy being adopted by Ares and try to rescue him or at least check on him regularly.
One possible plot twist could be Percy finding out that a fellow soldier is actually a spy from the enemy side. This would create a lot of internal conflict as he has to decide what to do about it.
Well, one plot twist could be that Percy discovers he has a hidden connection to one of the major Trojan heroes. Maybe he finds out he's related to Hector in some way, which makes his fight against the Trojans much more complicated. He might struggle with his loyalty to the Greeks.
A really interesting plot twist might be that Voldemort and Percy are forced to team up against a common enemy. They realize that there is a greater threat that neither of them can face alone. This would be a huge change in their relationship as they are initially enemies. Or perhaps Percy discovers that he has some latent wizarding abilities due to his godly heritage, which completely changes the dynamics of the fight as he can now use both his demigod powers and magic.
One possible motive could be self - defense. Maybe Ares was about to harm Percy's friends or family, and Percy had no other choice but to fight back and end up killing him.
One possibility could be a story where the daughter of Ares has to team up with Percy Jackson against a common enemy. Maybe they start off as rivals, given Ares and Poseidon's often - contentious relationship. But as they face challenges, they learn to respect each other's skills and eventually become friends. For example, they could be fighting against a powerful Titan who is trying to rise again and destroy the world of the demigods.