It really depends on the situation. If it's in a magical environment like Hogwarts, Harry might have an advantage with all the spells he knows. But if it's an open area, Ben with his Omnitrix could transform into an alien like Way Big and overpower Harry with his size and strength.
One great 'Ben 10' and 'Harry Potter' fanfiction is 'Ben Potter'. In this story, Ben Tennyson somehow finds himself in the wizarding world of Harry Potter. He has to deal with the magical threats there using his Omnitrix. It's a really interesting crossover as it combines the high - tech alien - transforming device from 'Ben 10' with the magic of 'Harry Potter'.
Yes, there might be. The idea of Harry Potter living in Big Ben is an interesting and creative concept for a fanfic. Writers often like to take familiar characters and place them in unusual settings to create unique stories.
There's a fanfiction where the Hogwarts houses are reimagined as different species from 'Ben 10'. So, Gryffindor might be like the Pyronites (the fire - based aliens in 'Ben 10'), known for their courage. Slytherin could be related to the Ectonurites (the ghost - like aliens), representing their sly nature. This reimagining creates a whole new dynamic within the Hogwarts world and also allows for some interesting interactions between the 'Ben 10' and 'Harry Potter' elements.
There might be a fanfiction where the characters in Harry Potter interact with Ben Drowned in a Hogwarts setting. For example, Harry and his friends could initially be suspicious of Ben Drowned's strange behavior and appearance. But as the story progresses, they realize that he may be a key to solving a mystery in the wizarding world, like a new dark magic that has seeped into the school and only Ben Drowned's unique perspective can help them figure it out.
One interesting crossover could be Ben using his alien forms to help the students at Hogwarts during a dark wizard attack. For example, his Heatblast form could be useful against ice spells. Another could be Harry Potter using magic to enhance Ben's Omnitrix. Maybe a spell that makes it transform faster or unlock new alien forms.
A common crossover concept is the idea of Ben attending Hogwarts. He would bring his street - smart attitude and his alien - fighting skills. His presence could disrupt the normal hierarchy at Hogwarts. He might get into trouble for using his alien powers instead of magic for certain tasks, but also prove to be a great asset when it comes to facing dark wizards. And his friends from the Ben 10 universe could potentially visit Hogwarts too, causing all sorts of chaos and adventure.
It's possible that a fanfic writer has imagined Harry Potter living in the Big Ben Tower. Maybe in this fanfic, Harry is on some sort of special mission in London and has to use the Big Ben Tower as a base or a hiding place. Or perhaps he discovers some magical secrets related to the tower. But without actively looking through fanfic databases, it's hard to say for certain whether such a fanfic exists.
Well, perhaps there's a magical artifact in the Harry Potter world that is malfunctioning and causing strange rifts in the universe. Ben 10 stumbles upon one of these rifts and ends up in Hogwarts. He teams up with Harry Potter and his friends. Ben realizes that his alien forms have different reactions to magic. Heatblast, for instance, can withstand some of the dark magic spells better than his human form, and together they embark on a journey to fix the artifact and close the rifts.
One great 'Ben Oc Harry Potter fanfiction' is 'The Lost Heir of Slytherin'. It has an interesting plot where the OC, Ben, discovers his hidden connection to Slytherin and has to navigate the politics and secrets within Hogwarts. Another one could be 'Ben's Magical Adventures' which focuses on Ben's unique magical abilities that are different from the normal Hogwarts curriculum.