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.
One of the main characters could be a Protoss high templar. This character could be the one who first discovers Harry and decides to take him under his wing. Another could be a Protoss zealot, who is initially skeptical of Harry but later becomes his ally in battles.
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 a 'Harry joins Avengers' fanfic, it could be really exciting. Maybe Harry would use his magic to help the Avengers in unique ways. For example, he could use spells to disarm villains quickly or create magical shields. His broomstick skills might also come in handy for quick getaways or aerial attacks.
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.
Harry could fit in by using his magic for reconnaissance. His invisibility cloak would be great for spying on enemies unnoticed. The Avengers could use this to their advantage when planning attacks.
A group of misfit soldiers could be main characters too. They are all a bit different and don't quite fit the typical military mold. But together with Harry, they form a unique unit. There might also be a high - ranking officer who takes an interest in Harry's progress and either helps or hinders him depending on the situation.