Well, it could be really interesting. Harry might use his magic to solve some of Smallville's strange problems that are usually dealt with in more 'normal' ways in the show. He could also introduce the people of Smallville to a whole new world of magic.
Another aspect could be the villains. Imagine Voldemort teaming up with one of Smallville's villains. It would be a huge threat to both the wizarding world and Smallville. The heroes from both sides would have to work together to defeat them, leading to some epic battles and interesting character interactions.
A possible fanfiction could be about a young wizard from Harry Potter's world who is a huge fan of the tales of Clark Kent in Smallville. This wizard accidentally casts a spell that transports him to Smallville. There he meets Clark and they become friends. Meanwhile, the presence of the wizard attracts the attention of some demons that Dante from Devil May Cry usually hunts. So Dante also arrives in Smallville, and they all have to work together to send the wizard back to his world and stop the demons.
Some characters might be very curious. Clark, for instance, would probably be interested in Harry's magic since he has his own superpowers. He'd want to know where Harry's powers come from and how they work.
You can start by looking on popular fanfiction websites such as Archive of Our Own (AO3). It has a large collection of fanfictions from various fandoms, and there's a good chance you'll find some 'Harry Potter Smallville' slash fanfiction there.
No. Harry Potter and Smallville are two completely different franchises. Harry Potter is about a boy wizard in a magical world, while Smallville is about the early years of Superman. There is no official connection between them in terms of one being a fanfiction of the other.
The magic in Harry Potter is often based on spells and potions. In fanfiction, when compared to the superpowers in Smallville, it's more about learned skills. For example, Harry has to study to cast spells, while Clark's powers are innate. Clark can fly and has super strength, which are physical abilities, while magic in Harry Potter can also affect the mind and create illusions.
If Harry Potter was a Smallville fanfiction, it would be a chaotic blend. The Hogwarts houses might be incorporated into Smallville's high school, and the Sorting Hat could be used to categorize the students. Harry could use his magic to enhance Clark's powers or to protect the town from threats that are both magical and related to Lex Luthor's schemes. But this is all just a wild and unrealistic idea as they are two very different stories.
Maybe Harry Potter could use his magic to solve some of Smallville's strange problems. For example, when there are meteor - related disasters in Smallville, Harry could use spells to protect the town. He might also meet Clark Kent and they could learn from each other. Harry could teach Clark about magic, and Clark could show Harry how to be a hero without relying solely on magic.
There could be a fanfiction where the magic of the Harry Potter world somehow seeps into Smallville. Maybe Hermione uses her knowledge to help Lana Lang with her scientific research, creating a unique blend of the two storylines. This kind of fanfiction often plays with the idea of how different fictional universes would interact if they collided.