An interesting plot might involve a prophecy that links the fate of both worlds. The prophecy states that only by combining the power of the Elder Wand and the sword of Aragorn at Helms Deep can the ultimate evil be defeated. Harry, along with Hermione and Ron, journey to Middle - earth. They face many challenges on the way. At Helms Deep, they have to work together with the people there. Harry uses his magic to enhance the strength of the defenders, while the others from Middle - earth contribute their combat skills. There could be a sub - plot where some of the characters from both worlds develop romantic interests in each other, adding an extra layer of complexity to the story.
Yes, there are likely fanfictions out there that combine elements of Harry Potter and the Battle of Helms Deep. Fanfiction writers often like to mash up different fictional universes. You can start by looking on popular fanfiction websites like FanFiction.net or Archive of Our Own.
One possible plot could be that Harry and his friends accidentally get transported to the Battle of Helm's Deep. They use their magic to turn the tide of the battle in unexpected ways. For example, Hermione could use a spell to make the walls of Helm's Deep even stronger, while Harry and Ron lead a group of wizards to fight off the orcs from above using broomsticks.
In a 'Harry Potter in a Custody Battle' fanfiction, it might be that Harry's muggle relatives and the wizarding community are in a custody battle. The Dursleys could claim they've been taking care of Harry all these years and have the right to keep him. But the wizarding world, like Hogwarts professors, might argue that Harry belongs with his own kind where he can learn magic and be safe from the threats only the wizarding world understands. There could be courtrooms filled with magic spells and arguments about what's truly best for Harry.
Perhaps in this fanfiction, the Battle of NY is a result of a magical artifact being discovered in NY that has the power to change the balance of the wizarding world. Harry and his friends, along with some American wizards they meet, have to race against time to keep it out of the hands of Voldemort or other dark forces. They face challenges not only from the bad guys but also from the muggle authorities who are starting to suspect something strange is going on.
One possible plot could be that Harry uses his magic in ways that are unexpected in the Middle - earth setting of Helm's Deep. Maybe he uses spells like 'Protego Maxima' to create a shield around the fortress, protecting it from the incoming orcs. He could also use the 'Accio' charm to bring weapons to the defenders when they are in need.
Obviously, Harry Potter himself would be a main character. He could bring his skills and courage from the wizarding world to the Battle of Helm's Deep. Hermione Granger would also be there. Her vast knowledge of magic could be crucial in finding solutions during the battle. Ron Weasley too, with his loyalty and his own brand of wizarding skills. Maybe Hagrid could also be involved, his huge size and strength being an asset in the physical combat at Helm's Deep.
One idea could be that Mewtwo somehow gets transported to the Harry Potter universe. It could be mistaken for a very powerful magical creature. Harry and his friends could initially be wary of it, but then realize it has no intention of harm and they team up to fight against the dark forces in the wizarding world using Mewtwo's psychic powers.
Another plot could be that Voldemort discovers Powergirl's existence and sees her as a threat. He tries to manipulate the wizarding world against her. But Harry and his friends believe in her and together they work to expose Voldemort's lies and also learn from Powergirl about different ways of fighting evil. Powergirl could introduce new concepts of justice and heroism to the wizarding world.
One possible plot could be that Harry discovers a hidden magical realm in the Pokémon world during one of his adventures. There, he meets Rayquaza. Rayquaza, being a powerful Pokémon, senses the strong magic within Harry. Together, they have to fight against a dark wizard who has found a way to corrupt Pokémon and use them for evil purposes in both the wizarding world and the Pokémon world.
Maybe the Timekeeper is actually a descendant of an ancient wizarding family who was tasked with keeping time in balance. In the fanfiction, Voldemort discovers this and tries to use the Timekeeper's powers to go back in time and prevent his own downfall. The Timekeeper has to fight against Voldemort's minions while also trying to find a way to protect the timelines. Harry and his friends get involved when they notice strange time - related disturbances around the school. They join forces with the Timekeeper to stop Voldemort's evil plan.