I think it would be really interesting to focus on Ron Weasley and The Flash. Ron has a laid - back yet brave personality. The Flash has his super speed. Ron could be in awe of The Flash's speed and The Flash could be amused by Ron's wizarding ways. They could bond over their shared sense of humor. Also, Dumbledore and Martian Manhunter could be a fascinating pair. Dumbledore with his vast wisdom and magical powers and Martian Manhunter with his telepathy and shape - shifting abilities. They could communicate on a deeper level, sharing their knowledge of different universes and different forms of power.
Hermione Granger and Batman would be great to focus on. Hermione is extremely intelligent and well - read in the magical lore. Batman is a brilliant detective with a vast knowledge of various sciences and combat skills. Their interaction could be really engaging. Hermione could introduce Batman to the world of magic, and Batman could show Hermione how to use her intelligence in a more practical, non - magical way in different situations. Also, Wonder Woman and Ginny Weasley could be an interesting pair. Wonder Woman with her Amazonian strength and Ginny with her powerful magic could team up and have some great adventures together, sharing their different perspectives on heroism.
Definitely Alex Mercer. His powers are so unique compared to the Justice League members. He can shapeshift into anyone, has superhuman strength, and can regenerate from almost any injury. This would create a lot of interesting interactions with the League. For example, he could transform into Superman to test the limits of his own power against the real Superman, which would lead to a really cool showdown.
Definitely Superman and the Red Ranger. Superman is known for his incredible strength and moral compass, and the Red Ranger is often the leader of the Power Rangers. Their leadership styles could either clash or complement each other. Also, Batman and the Blue Ranger would be fascinating. Batman's detective skills and strategic planning could be contrasted with the Blue Ranger's more technical skills in handling the Ranger technology.
Batman would be really interesting. He's a master detective and strategist. He would be the one to study the Predator's behavior and try to find its weaknesses. His lack of superpowers but high intelligence would make for an interesting dynamic against the technologically advanced and physically powerful Predator.
Wonder Woman and Ron Weasley could be an interesting combination. Ron is often underestimated but has a lot of heart and loyalty. Wonder Woman, with her strength and noble character, could inspire Ron to be more confident in himself. Ron could introduce Wonder Woman to the fun and sometimes chaotic side of the wizarding world. They could work together to fight off enemies, with Ron using his magical skills and Wonder Woman using her combat and leadership skills.
Leonidas from '300' would be a great character to focus on. Imagine his reaction to the magic of the Harry Potter world. He's a fearless leader, and seeing him interact with wizards and magical creatures would be fascinating. He might see the magic as a new weapon or a strange form of sorcery that he has to either embrace or fight against.
Miss Martian from Young Justice could be a great focus. Her powers of telepathy and shape - shifting make her a formidable opponent. Also, her experiences as a young hero trying to fit in and be accepted, especially when facing the more established Justice League, can lead to some really interesting character development and plot twists.
Luna Lovegood would be an unexpected but fascinating choice. Her unique perspective and belief in all kinds of magical creatures could make her very open - minded about the vampires and werewolves in the Twilight world. She might see them as just another type of magical being and try to find common ground, which could lead to some really interesting interactions and storylines.
Batman would be a great character to focus on. He doesn't have superpowers like the others but his intelligence and strategic mind are his greatest assets. In a 'Justice League Infinity War' fanfic, he would be the one trying to figure out the complex nature of the Infinity Stones, coming up with plans to outwit Thanos. He might also have some tense moments where he has to rely on the super - powered members of the League in ways he's not used to, which could lead to some great character growth and interesting interactions.
Flash is always an interesting character. His super - speed gives him a unique perspective and allows for some really cool action scenes. You can explore how he perceives time differently during battles.
Nico di Angelo would be an interesting choice. His dark and brooding nature along with his powers over the dead would be a contrast to the more light - hearted aspects of the Harry Potter world in some ways. He could introduce the wizards to the concept of the Underworld and the spirits. And his ability to shadow - travel could be used in some really creative ways, like getting the group in and out of dangerous situations quickly, whether it's in Hogwarts or in the magical areas related to Greek mythology.