Perhaps Harry is mistaken for a god by the Spartans due to his magical abilities. He has to navigate this newfound status while also trying to find a way back to his own world. He could use his position to introduce some new ideas to the Spartans, like better medical magic - inspired treatments for their wounded warriors, all while trying to keep his true identity a secret.
A possible plot could be that Voldemort also finds his way into the Spartan world. Harry then has to form an alliance with the Spartans to fight against Voldemort. The Spartans, with their combat skills, and Harry, with his magic, combine forces. Harry might teach some of the Spartans basic defensive spells to protect their city - state from Voldemort's dark magic attacks.
One idea could be the warrior aspect making Harry a more battle - hardened and strategic fighter. Maybe he trains in a secret ancient magic that combines the wizarding and warrior skills, allowing him to face Voldemort's forces in a completely new way.
One idea could be the Holy Warrior having unique magical abilities that are different from the regular magic in the Harry Potter world. For example, they might be able to purify dark magic on a large scale.
A plot could involve Harry being trained in an ancient art that combines mage and warrior techniques. He then has to use these skills to protect Hogwarts from a new threat that is immune to normal magic. He might have to physically fight and use magic at the same time, like creating magical shields while engaging in hand - to - hand combat.
A story where the Warrior Princess is actually a long - lost heir to a powerful magical family in the Harry Potter universe. She discovers her heritage while at Hogwarts and has to balance her newfound magical powers with her existing warrior skills. She could form an unlikely alliance with some of the main characters to fight against a dark force that is interested in her unique combination of powers.
One idea could be to have a wizard from the Harry Potter world time - travel back to the era of ancient warriors. They could use their magic to aid or disrupt the battles of the ancient warriors, creating a whole new dynamic in the historical events. For example, a wizard could help an outnumbered group of ancient warriors by using magic to create illusions to confuse their enemies.
Harry's magic would likely be both a source of wonder and suspicion in Spartan culture. His ability to conjure things out of thin air might be seen as a gift from the gods by some, while others would be more cautious. For example, his magic could be used to help in the Spartan's agricultural efforts, like making crops grow faster, but this might also disrupt their traditional ways of farming.
Another idea is that a Spartan 3 has a family connection to one of the demigods in Percy's world. This connection leads to a series of events where they have to protect a sacred artifact from both the Titans and the monsters. The Spartan 3's training and the demigod's powers would be combined to fend off the enemies.
In Spartan Game of Thrones fanfiction, a possible plot is the discovery of Spartan artifacts in Westeros. A young maester finds a hidden vault with Spartan weapons and armor. He tries to understand their purpose and power. Meanwhile, a knight from a minor house gets hold of one of the Spartan swords. With it, he gains an edge in combat and becomes a key player in the power struggles, attracting the attention of the major houses and changing the course of the political and military affairs in Westeros.
One idea could be that the scarecrow comes to life and becomes a magical creature that helps Harry Potter in a dangerous task. Maybe it has the ability to sense dark magic and warns Harry of approaching threats.
Perhaps Orochi is a hidden figure in the history of the wizarding world that has been forgotten. Harry discovers some old texts in the Hogwarts library that mention Orochi in relation to a long - lost magical artifact. This leads him and his friends on a quest to find the artifact before it falls into the wrong hands, as it has the power to either control Orochi or be used by Orochi to gain ultimate power over the wizarding world.