One possible plot could be that Hogwarts students are transported back in time to the era of the Roman Legion. There, they discover that magic was used in different ways during that time. For example, wizards in the Roman Legion might have used magic for battle strategies, like creating illusions to confuse the enemy. Harry and his friends could then learn these ancient magical techniques and bring them back to their own time to use against Voldemort.
A great plot could be that a dark wizard in the past, during the Roman Legion days, had a powerful and dangerous spellbook. The spellbook gets lost over time and is found in the present - day Hogwarts. Harry, Ron, and Hermione start to investigate its origin and find out that they need to travel back in time to the Roman Legion era to stop the dark wizard from ever creating this book. Along the way, they face challenges like dealing with Roman soldiers who are suspicious of their strange powers and trying to fit into the society of that time.
The characters would have to adapt in many ways. For instance, Harry would need to get used to the strict discipline of the Roman Legion. He's used to the rules at Hogwarts, but the military - style discipline would be a whole new level. Hermione would likely be fascinated by the knowledge systems in place in the Roman Legion. She might study their strategies and try to incorporate them into her magical knowledge. Ron, on the other hand, would probably struggle a bit with the physical demands at first, but he'd learn to keep up as he did during his adventures at Hogwarts.
Well, in some fanfictions, they might explore the backstories of minor characters from the 'Harry Potter' series. Like, there could be a story centered around a house - elf who has a secret power. Or perhaps a story where a character from the legion discovers a hidden prophecy that changes the course of the wizarding world.
One possible plot could be that Batman is somehow transported to the future where the Legion of Superheroes exists. He has to adapt to their high - tech world and their different ways of handling crime. At the same time, the Legion could be facing a new threat that has some similarities to the villains Batman has faced in his time. So, Batman uses his detective skills and strategic mind to help the Legion out.
Some 'Harry Potter Roman Empire fanfiction' might focus on the characters from Harry Potter being reimagined as Roman citizens. Hermione could be a brilliant scholar in a Roman school of magic, Harry might be a gladiator - like figure fighting dark magic in the Roman arenas. Or perhaps Voldemort has ties to an ancient Roman dark cult and the Trio has to uncover this connection to defeat him. These kinds of stories blend the two very different worlds in unique ways.
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.
In a fanfiction, the Triforce could be split among Harry, Ron and Hermione. Harry gets the Triforce of Courage, Ron the Triforce of Power (which he learns to use in a more responsible way), and Hermione the Triforce of Wisdom. Together, they combine their powers of the Triforce with their magic skills to defeat Voldemort once and for all. The Triforce's power could also be used to reform the wizarding world, bringing more equality and peace.
Well, perhaps the magical creatures from the Harry Potter universe end up in the '300' setting. Dragons could be used by the Spartans to fight their enemies. The centaurs could also join the battle, lending their archery skills. And Harry could learn some combat skills from the Spartans, which he could then use back in the wizarding world to fight Voldemort's Death Eaters.