It could be amazing. Imagine Clark Kent from Smallville finding himself in the dark and gritty Gotham. He might use his powers to help Batman clean up the city in a different way. Their different moral compasses could lead to some interesting interactions, like Clark's more idealistic view clashing with Batman's darker pragmatism.
A Smallville - Gotham crossover fanfiction could involve the villains as well. Lex Luthor could team up with some of Gotham's rogues gallery. This would force Superman (from Smallville) and Batman to work together more closely. They would have to combine their skills, Superman's super strength and flight with Batman's detective skills and gadgetry, to take on the combined threat.
Lex Luthor would see Gotham as a new playground. He could try to manipulate some of the power players in Gotham, like the mob bosses or corrupt politicians. Batman would be on to him quickly, though. And this would lead to a cat - and - mouse game between Lex and Batman. Meanwhile, Clark would be caught in the middle, trying to figure out who to trust and how to stop Lex without causing too much collateral damage.
One possible plot could be Naruto being transported to Gotham. He might initially be confused by the dark and crime - ridden city. But with his ninja skills, he could start to fight crime in his own unique way, perhaps using his shadow clones to patrol different areas of Gotham simultaneously. His positive and determined attitude could also inspire some of the more down - trodden citizens of Gotham, like the orphans in the city.
An alien entity that has been dormant beneath Smallville for centuries could awaken. This entity could view Chloe and Clark as threats to its plans of taking over the town and the world. It could use its advanced powers to manipulate the townspeople and turn them against Chloe and Clark, making it a formidable foe.
The Riddler could be a great main antagonist. He could create puzzles and traps that are based on both the Flash's speed and Gotham's dark secrets. For example, he might set up a trap where the Flash has to run through a maze of moving walls in Gotham while also solving riddles that are related to Batman's past.
A new meta - human villain created specifically for the crossover could be interesting. This villain could have the power to control shadows and use them to move quickly, mimicking the Flash's speed in a way. In Gotham, he could use the city's many dark corners and alleys to his advantage. He could also be a master of disguise, fooling both the Flash and Batman. His motives could be to take over both Gotham and Central City and create a new world order where meta - humans rule over normal humans.
The ability to reimagine character relationships is also unique. Characters who might not have interacted in their original shows can form new alliances or rivalries in smallville crossover fanfiction. For instance, a character from Smallville could have a completely different relationship with a character from 'Supergirl' in a crossover. This gives fans a chance to see their favorite characters in new lights and explore different emotional arcs.
I think Rogue would be an interesting main character in a crossover. Her power - absorbing abilities could create some really unique situations, especially when she comes into contact with Clark or other powered individuals in Smallville. On the Smallville side, Lex Luthor could be a main character too. His devious nature and thirst for power could put him at odds with both the X - Men and Clark, making for a complex and engaging story.
The clash of different mythologies. Smallville has its Kryptonian and small - town - hero story, while Supernatural has demons, angels, and hunters. When these two collide in a fanfic, it creates a really unique and exciting world. For example, seeing how Clark's superpowers interact with the magic and weapons of the Winchester brothers.
One key element is the character interactions. Seeing characters like Clark Kent and Oliver Queen interact, given their different backgrounds and powers, is really exciting. Another element is the merging of storylines. For example, how the threats in Smallville might be related to those in the Arrowverse.
A common theme is the clash of two different worlds. Smallville is more of a small - town superhero origin story, while Buffy deals with the supernatural in a more urban and 'chosen one' kind of way. So, the fanfics often explore how these two concepts collide.