One interesting 'Harry Potter Alice' fanfiction could be about Alice from Wonderland somehow ending up in the wizarding world of Harry Potter. Maybe she uses her unique Wonderland abilities to solve some of Hogwarts' mysteries.
In some fanfiction, Harry could be the one who enters Wonderland. Maybe he's on a quest there, and he meets the Cheshire Cat. The Cheshire Cat could guide him, but in a very mysterious way, just like how Dumbledore often gives cryptic advice to Harry. Harry would have to adapt his magical skills learned at Hogwarts to the strange magic of Wonderland.
A really interesting plot would be if the characters from Alice in Wonderland were students at Hogwarts. Alice could be in Gryffindor, always getting into strange adventures around the school that remind her of Wonderland. The Mad Hatter could be a Potions master who makes the most bizarre potions. And the Red Queen could be a strict teacher who is always trying to catch students breaking the rules. There could also be a plot where Voldemort tries to take over Wonderland for some dark purpose, and Harry and his friends have to stop him there.
One idea could be that a male character from the Harry Potter world meets Alice (presumably from Wonderland in a crossover). Maybe he stumbles upon her world while exploring magical realms and they have to team up to solve a mystery that links both their worlds, like a dark magic that is seeping from Wonderland into Hogwarts.
One interesting crossover could be the magic systems. In Gakuen Alice, the students have unique 'Alice' powers, and in Harry Potter, there are different types of magic. A crossover could explore how characters from both worlds interact with each other's magic. For example, a student with the 'Fire Alice' could team up with a Gryffindor student to fight a common enemy using their combined powers.
A great fanfic idea could be a crossover where Voldemort tries to take over Wonderland. The characters from Wonderland, along with the help of the 'Harry Potter' gang, have to fight against him. The magical creatures in Wonderland, like the Jabberwocky, could team up with the magical creatures from the 'Harry Potter' universe, such as hippogriffs, to defeat Voldemort.
One idea could be that Harry discovers a hidden magical connection to the vampire world through Alice. Maybe Alice has a premonition about Harry's role in a new, greater threat that involves both the wizarding and vampire realms. They then have to work together to figure out how to stop it.
One interesting Harry Potter fanfiction could be those that explore alternate universes, like 'The Harry Potter Who Lived'. It shows a different path for Harry where he makes different choices. Another great one might be fanfics that focus on the backstories of minor characters, for example, a story about Neville's parents before they were tortured. There are also fanfictions that pair unexpected characters in relationships, which can be really fascinating to read.
One interesting Harry Potter fanfiction could be a story that explores what would happen if Harry had been sorted into Slytherin instead of Gryffindor. It could show his struggle to fit in with the Slytherin values while still holding onto his own sense of right and wrong.
One interesting Harry Potter fanfiction could be about an alternate universe where Harry is sorted into Slytherin. It shows a different side of his character development as he has to navigate the house known for its cunning. Another great one might be a story that focuses on the Marauders' time at Hogwarts in more detail, exploring their pranks and friendships deeper.
One interesting Harry Potter fanfiction could be a story where Harry discovers a hidden magical ability that was not explored in the original books. For example, the power to communicate with magical creatures on a deeper level, allowing him to gain new allies in the fight against Voldemort.