Scooby - Doo and the gang. It would be a really unexpected and fun crossover. The Mystery Inc. could accidentally stumble into the Justice League's world while investigating a mystery. They could end up helping the League in a way that no one expected, like finding a hidden clue that was overlooked by the super - powered heroes.
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.
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.
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.
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 Naruto would be the most interesting. His boisterous personality and powerful ninja abilities would stand out in the Justice League world.
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.
Batman and Iron Man would be a fascinating pairing. Both are brilliant inventors and strategists. They could compare their high - tech gadgets and suit designs. Batman's detective skills paired with Iron Man's scientific knowledge could make them a formidable duo in solving the mystery of some cross - universe threat. They might also argue about who has the better tech, which would be quite entertaining.
In my opinion, it could be Scarlet Witch. Her reality - warping powers are extremely potent. In a crossover, she could potentially change the very fabric of the combined universes. If she were to go rogue or be influenced by an evil force, she could cause chaos on a scale that would be very difficult for the combined might of the Justice League and the Avengers to handle. Even Superman's invulnerability might not protect him from her reality - warping abilities. She could simply make him mortal or take away his powers altogether.
The interaction between Superman and Supergirl would be really interesting. They share the same Kryptonian heritage but have had different experiences on Earth. Superman could act as a mentor to Supergirl, or they could have a friendly competition to see who is stronger in different situations.
Definitely Superman and Master Chief. Superman has all his superpowers like super strength, flight, and heat vision, while Master Chief is a highly trained super - soldier. They could have an interesting dynamic, with Superman respecting Chief's combat skills and Chief being in awe of Superman's powers. They could work together to take on powerful enemies, like Darkseid if he somehow got involved in the Halo - Justice League crossover.
Rorschach and Batman would also be a great pair. They are both dark, brooding characters who are very dedicated to their own versions of justice. They might start off clashing due to their different methods, but eventually find common ground in their pursuit of truth.