Perhaps there could be a school in the Hetalia world that is similar to Hogwarts. Students from both worlds are invited to study there for a semester. The Harry Potter students are shocked by the different ways of teaching magic in the Hetalia school, and there are some friendly rivalries between the students. There could also be a mystery at the school that they all have to solve together.
One idea could be a Quidditch tournament where the Hetalia characters represent different magical countries. For example, Italy could be on a team and use his charm to distract the opposing team. Another could be a magical artifact that gets lost in the Hetalia world and the Harry Potter characters have to go there to retrieve it. Maybe they have to deal with the political situations in the Hetalia world while searching for the artifact.
One idea could be to have the characters from Hetalia and Homestuck participate in an international event together. For example, a global gaming tournament where the strategic minds of the Hetalia characters and the unique abilities of the Homestuck characters collide. Another plot could involve a dimension - hopping adventure where the Hetalia countries find themselves in the Homestuck universe and have to adapt to its strange rules and customs while trying to find their way back home. Or perhaps a story where the trolls from Homestuck end up in the Hetalia world and start causing chaos among the different nations.
One idea could be a sports competition between the characters from Hetalia and Countryhumans. They could compete in various events like a grand international Olympics - style tournament. This would bring out their competitive sides as well as show their national pride. Another plot could be a diplomatic crisis where the characters from both universes have to work together to solve a global issue, like a sudden environmental catastrophe that affects all of their 'countries'.
One idea could be that the characters from Hamilton are transported to the Hetalia world. Hamilton could use his political acumen to influence the international relations in the Hetalia universe. For example, he might try to introduce new democratic ideas among the nation - states there.
One idea could be that the countries from Hetalia are somehow transported into the Naruto world. For example, Italy could end up in Konoha and his carefree attitude might amuse or annoy the ninja there. Maybe he gets involved in a mission by accident and his unique skills from being a nation, like his knowledge of different cultures and languages, could come in handy. Another plot could involve Naruto being sent to the Hetalia world and having to deal with international relations in a very different way than he's used to in the ninja world. He might try to use his ninja way to promote peace between the nations there.
Maybe the Black Butler characters are sent to the Hetalia world. Sebastian could be intrigued by the different cultures and politics of the countries. He might use his charm and skills to manipulate some of the smaller countries for his own or Ciel's gain. Grell could go wild with excitement over all the new 'pretties' he could potentially reap in this new world filled with so many different characters. And Ciel would be constantly trying to figure out how to get back to his own world while also dealing with the strange international relations in Hetalia.
Well, there could be a story where a powerful dark magic object from the wizarding world is brought to Sunnydale. Buffy discovers it and realizes it has the power to disrupt the balance between good and evil in her world. She then travels to Hogwarts to seek help from Harry Potter. Along the way, she meets various magical creatures and has to navigate the wizarding world's politics. Once she meets Harry, they discover that there is a common enemy behind all this, someone who wants to use the chaos in both worlds for their own gain, and they must work together to stop this mastermind.
One idea could be a time - travel plot where a character from Legacies goes back in time to the era of Harry Potter. They could interact with the young Harry and his friends, and perhaps help in the fight against Voldemort in unexpected ways. For example, they might introduce a new spell from the future that gives the Order of the Phoenix an edge.
One idea could be that the Grimm creatures start to appear in the wizarding world. Harry and his friends have to use both magic and knowledge of Grimm lore to fight them. For example, they might discover that some dark magic has opened a portal between the two universes.
One idea could be that the Order of the Phoenix teams up with the Kingsman. The wizards could use some of the Kingsman's high - tech gadgets and fighting skills, while the Kingsman could learn about magic. For example, Harry Potter could be trained by a Kingsman agent in hand - to - hand combat, which would give him an edge in battles against Death Eaters. Another plot could involve a villain who is a hybrid of a dark wizard and a criminal mastermind that the Kingsman usually deal with. The two groups would have to combine their resources to stop this new threat.
Maybe some of the Hogwarts students are sent to the Naruto universe. They would be amazed by the ninja techniques. Hermione, for instance, would be eager to study how chakra works and might even find a way to combine it with magic. Harry could learn some ninja stealth skills which would be useful in his fight against Voldemort.