Maybe Harry gets tired of the regular magical schools and discovers a hidden, more specialized fourth school. There he could meet new friends and face different kinds of magic challenges.
He could meet new professors who have unique teaching methods and vast knowledge of different magics.
Well, perhaps Harry could discover a magical portal that leads him to the Protoss world. There, he might learn their advanced technology and unique culture. Maybe he uses his magic in combination with Protoss powers to fight off some common enemies.
Well, one possibility could be that Harry, through some sort of magical accident or powerful spell, gets transported to the world of Loki. Maybe he initially mistakes Loki for a powerful wizard and tries to seek his help to get back home. Loki, being the trickster he is, might see Harry as an interesting pawn in his schemes against the Asgardians or other foes. Harry, with his own magical skills from the wizarding world, could end up being both a hindrance and a help to Loki, leading to all sorts of adventures and misadventures as they navigate their two very different magical realms.
One possibility is that Harry joins SHIELD after realizing the magical world can't fully contain the threats he's faced. In the fanfic, he could be a key figure in bridging the gap between the magical and non - magical worlds within SHIELD. He might introduce magic - based security measures or use his ability to detect dark magic to identify threats that normal technology can't pick up.
One possible plot for 'harry joins the military fanfic' is that Harry joins the military to find a new purpose in life. He comes from a troubled background and sees the military as a way to discipline himself. He starts in a basic training unit where he meets all kinds of different people from different walks of life. As he progresses, he discovers a hidden conspiracy within the military that threatens national security. Harry then uses his wits and new - found military skills to try and expose the conspiracy and save the day.
In the 'harry joins voldemort fanfic', it could start with Harry being captured by Voldemort. Instead of killing him immediately, Voldemort decides to brainwash him. Through a series of dark magic spells and psychological manipulation, Harry starts to see Voldemort as a mentor. As time passes, Harry becomes an integral part of Voldemort's inner circle, helping him plan attacks against the wizarding world, believing that they are actually for a greater good as Voldemort has convinced him.
The plot could involve Harry using his magic in the Stargate universe. Maybe he could use spells to help the Stargate teams in their battles against the Goa'uld. For example, a simple Stunning Spell could be very useful against the Jaffa warriors. He could also use magic to decipher some of the ancient technologies that the Stargate teams come across, since he has experience with ancient magic in his own world.
One possible plot could be that Harry is brought into the SG1 world because of a magical accident involving the Stargate. Once there, he realizes that there are dark forces at work in this universe too. He teams up with SG1 to find a way to get back home while also helping them to stop an impending invasion. Harry uses his Patronus charm to fend off some of the scarier alien creatures, and his knowledge of potions comes in handy when they need to cure a strange alien disease.
The plot might revolve around Harry's adaptation to the military lifestyle. He'd have to get used to the strict discipline, early mornings, and intense physical training. Along the way, he might discover a hidden talent for air combat strategy. And there could be a sub - plot about his relationship with his family back home who are both proud and worried about him.
In such a fanfic, perhaps Harry Potter is magically manipulated. The Dark Lord could use some powerful dark magic to cloud Harry's judgment. Harry might then start to use his skills and knowledge, which were originally meant for good, to help the Dark Lord's evil plans. For example, he could use his Parseltongue ability to communicate with dark creatures for the Dark Lord's purposes.