How about Snoopy from Peanuts? He could fly his doghouse into Hogwarts. The Hogwarts teacher would be so confused at first. But Snoopy could have some kind of special dog - related magic that the teacher could try to understand. Maybe Snoopy could help in the Care of Magical Creatures class in a very unexpected and comical way.
I think Groot from Guardians of the Galaxy would be really unexpected. Imagine a Hogwarts teacher trying to communicate with a talking tree. It would be hilarious and full of misunderstandings at first. But then they could find a way to work together, maybe Groot could use his strength to help with some physical tasks around Hogwarts in a really unique way.
The Doctor from Doctor Who. He could bring his time - traveling TARDIS into the Justice League's universe. His wits and knowledge of different times and places could be a huge asset in solving a mystery that the Justice League is facing, and his alien nature would add an interesting element.
Gandalf from 'The Lord of the Rings'. His magic and wisdom could add a new dimension to the story. He could help the X - Men in their fight against Apocalypse, perhaps using his magic to counter some of Apocalypse's powers or providing strategic advice.
I think Gizmo from the Teen Titans could be really unexpected. He's used to dealing with high - tech and more colorful villains. Throwing him into Arkham Asylum, with all its dark and gritty atmosphere and dangerous inmates, would create a really interesting dynamic. He could try to use his gadgets to either escape or try to take over the asylum, which would be hilarious considering the much more menacing villains there.
Mario. The plumber from the Mushroom Kingdom in a crossover with Final Fight would be quite a shock. However, his acrobatic jumps and ability to use power - ups could add a unique element to the fanfiction. He could jump on the heads of the Final Fight thugs or use a fire flower to defeat them in an unorthodox way.
Wall - E could be a very unexpected character. He's from a post - apocalyptic world where he's all alone collecting trash. In an Ever After High crossover, he could land in the school's garden and start cleaning up. The students would be so confused by this little robot. But he could also teach them about the importance of taking care of the environment, which is something not really focused on in the fairy - tale world of Ever After High.
I think Groot from Guardians of the Galaxy is already quite an unexpected character in the Infinity War story. But if we want to be even more out - there, how about Snoopy from the Peanuts? Imagine Snoopy flying his doghouse into the middle of the battle against Thanos and using his imagination - powered antics to distract the villains. It would be a really wacky and unexpected addition.
A possible plot could involve a Hogwarts teacher from the past crossing over to the present day Hogwarts in a fanfiction. This could lead to interesting clashes between old and new teaching methods. Or perhaps a crossover with a character from a fairy - tale world. The teacher could help the fairy - tale character understand the more complex magic of Hogwarts.
The Doctor from Doctor Who would be an interesting addition. He could land his TARDIS in the jungle and get involved in the jungle's affairs. He might help Mowgli and the animals deal with a time - related threat, like a time - traveling villain who wants to change the history of the jungle.
The Care Bears. It would be really unexpected to see the cute and cuddly Care Bears in the dark and violent world of Twisted Metal. Maybe they could be there to try and bring some peace and love, but end up getting caught in the crossfire between the Twisted Metal drivers.
How about a character from a classic fairy tale? For instance, Cinderella could be accidentally brought into the 'The Loud House' world during the crossover. Her presence could contrast with the chaos of Lincoln's bad luck. She might try to use her positive attitude and her experience with hardships to help Lincoln deal with his situation.