In a Hetalia and Naruto crossover fanfiction, a cool plot could be that there is a tournament. Characters from both universes participate. The Naruto characters show off their jutsu and fighting skills, while the Hetalia characters use their diplomatic wits and any unique powers they might have. It could be a way to promote understanding between the two groups. For instance, America from Hetalia might be overconfident at first but then learn a lot from the Naruto characters about discipline and hard work. And Sasuke could learn about different cultures from the Hetalia characters during the tournament.
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.
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 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 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'.
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.
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 that the Hetalia characters suddenly find themselves in the BNHA universe and have to adapt to having quirks. For example, Italy might be really confused at first but then find that his ability to make good pasta could be turned into a quirk related to food creation or manipulation. Another could be that the BNHA characters visit the Hetalia world and are shocked by the different cultures and historical settings. They could learn about different forms of power and leadership from the Hetalia characters.
One idea could be to have Sherlock investigate a mystery related to a historical event in Hetalia. For example, he could look into the disappearance of a valuable artifact during a time of war in one of the Hetalia countries. Another idea is to have the Hetalia characters assist Sherlock in solving a case in London. Maybe Italy could provide some unexpected insights with his unique perspective. Also, a crossover could involve Sherlock being transported to the Hetalia world and having to adapt to its strange rules and cultures while still using his detective skills to solve local mysteries.
Maybe there's a plot where a powerful and rare Pokemon is causing chaos in the Naruto world. The Naruto characters have to team up with some expert Pokemon trainers from the Pokemon world to stop it. Naruto could learn about the unique bond between trainers and Pokemon and apply some of those concepts to his relationships with his friends. And the Pokemon trainers could be in awe of the ninja techniques and use some of that knowledge to improve their own strategies.
A possible plot is that the Zoids themselves gain some of the abilities of Naruto characters. Imagine a Zoid being able to use chakra and perform ninjutsu. A Fire - type Zoid could use the Fireball Jutsu. This would completely change the dynamics of Zoids battles. Also, the story could revolve around a group of Naruto characters trying to stop an evil force in the Zoids world that is using Zoids in a way that could destroy both the Zoids world and the Naruto world. They would have to learn about Zoids and team up with local Zoid pilots to fight against this common enemy.
One idea could be a character from Naruto ending up in the Descendants' world. For example, Naruto himself could be transported there. He might use his ninja skills to deal with the challenges in the Descendants' world, like facing off against the villains in a very different way compared to the Descendants' characters. Maybe he'd use his shadow clone jutsu to confuse the bad guys or his Rasengan to take on powerful foes.