One possible plot could be that Ares, the god of war from Greek mythology, decides to test the courage of the young wizards at Hogwarts. He creates various dangerous challenges within the Forbidden Forest. Harry Potter, being the brave hero, steps up to face these challenges, with his friends by his side. Along the way, they discover new spells and powers that are somehow related to Ares' influence.
Well, perhaps in this fanfiction, Ares is intrigued by the magic in the Harry Potter world. He appears at Hogwarts and offers a deal to Harry. If Harry can win a series of battles, Ares will grant him a powerful magical artifact. Harry, being Harry, accepts the challenge. He has to use his wits and the magic he has learned to compete in these battles which are designed by Ares with his war - like mindset. The story could then follow Harry's journey through these battles and how it affects his relationships with his friends and teachers.