Rapunzel herself could be very interesting. Her kind - hearted and curious nature would make her a great addition to the Little Mermaid's world. She could bond with Ariel over their shared sense of adventure and their experiences with magic. Plus, her hair could have all sorts of interactions with the underwater environment.
I think Flynn Rider would be really interesting. Imagine him in the underwater world of the Little Mermaid. His charm and roguish ways would be a contrast to the more innocent mermaids. He could probably teach Ariel a thing or two about being a bit more street - smart while also being charmed by her naivete.
Hermione. She's intelligent and resourceful. In the Tangled world, she could use her knowledge of spells and potions to solve problems. For example, she could brew a potion to make Rapunzel's hair grow even longer and stronger if it was damaged in some way.
In Tangled and Little Mermaid fanfiction, Pascal, Rapunzel's chameleon, can be a really fun character to include. He could have some really interesting interactions with Flounder or Sebastian from The Little Mermaid. Maximus, the horse from Tangled, could also be a great addition. And of course, Ursula from The Little Mermaid is a popular villain to feature. Writers can create new scenarios where the heroes from Tangled have to face off against her.
One idea could be a crossover where Rapunzel from Tangled ends up in the underwater world of The Little Mermaid. Maybe she uses her long hair to help Ariel in some way, like rescuing her from a sunken ship. Another idea is that Flynn Rider and Prince Eric meet and have to work together to save both their princesses from an evil sea witch who has joined forces with a land - based villain.
War from Darksiders could also be a great crossover character. His brute strength and his weapon skills would be a contrast to the ninja in the Naruto world. He could be sent to the Naruto world on a mission and end up getting involved in the conflicts between the ninja villages. His war - like nature might make him either a feared or respected figure among the ninjas.
One idea could be that Rapunzel's long hair has some sort of magical connection to the underwater world of the Little Mermaid. Maybe her hair can be used to communicate with the mermaids or even has the power to purify the polluted parts of the ocean.
One interesting Tangled and The Little Mermaid fanfiction could be about Rapunzel and Ariel meeting. Maybe Rapunzel's long hair gets tangled in Ariel's underwater world. It could be a story of them helping each other out of difficult situations and learning about each other's very different worlds.
I think Naruto himself would be very interesting. His energetic and fearless personality would contrast greatly with the spooky atmosphere of Sleepy Hollow. He would likely approach the strange happenings in Sleepy Hollow with his usual enthusiasm and determination to overcome any challenges.
I think Geralt would be really interesting. He has such a different background and set of skills compared to the characters in Harry Potter. His no - nonsense attitude and his experience with all kinds of monsters would make for some great interactions with the Hogwarts students and faculty. He could be a bit of an outsider, but also a valuable ally in the fight against dark forces.
Definitely Naruto himself. His positive attitude and never - give - up spirit would contrast greatly with the more morally ambiguous characters in Highschool DxD. He could also introduce the concept of chakra and ninja techniques, which would be completely new to the Highschool DxD world. His rasengan, for example, would be a very powerful and unique ability in that universe.
Eleven could also be a great crossover character. Her telekinetic powers are very different from the abilities in the Naruto universe. If she were to be in the Naruto world, she could learn about chakra and perhaps find new ways to use her powers, or vice versa in the Stranger Things world where Naruto characters show up.