In a 'Harry Potter is Vegeta fanfiction', it could be that Vegeta's personality traits are transferred to Harry. So Harry becomes more aggressive and prideful like Vegeta. He might view the wizarding world's challenges in a different light, seeing them as opportunities to prove his superiority. His relationships with his friends could also change. Instead of relying on teamwork in the traditional wizarding way, he might take on more of a lone - wolf approach, like Vegeta often does. And in battles, he could use Vegeta - inspired fighting stances and techniques, combined with his magic, to defeat his foes in unexpected ways.
There could be a really interesting fanfiction where Harry Potter, Vegeta from Dragon Ball Z, and Piccolo end up in a stadium. Maybe they are forced to compete in some sort of magical - martial arts tournament. Harry could use his magic skills, like the Patronus charm to fend off attacks, while Vegeta and Piccolo show off their powerful energy blasts and combat techniques. The stadium would be filled with excited fans from all their respective universes, creating a really chaotic but exciting scenario.
One way is to have a magical accident. Maybe a wizard in the Harry Potter universe was experimenting with interdimensional travel and accidentally brought Vegeta and Piccolo to the stadium in the wizarding world. Then they all have to figure out how to get back to their own worlds while dealing with the chaos that ensues in the stadium, like angry fans or other magical creatures that are attracted to the strange energy.
Well, it would be really strange and interesting. Vegeta is from Dragon Ball, a very different universe from Harry Potter's. If Harry was raised by Vegeta, he might be trained in combat from a very young age, like how Vegeta trains Trunks. He could be more aggressive and less reliant on magic wands, perhaps using his ki powers instead. And his attitude towards enemies would be more like Vegeta's, full of pride and determination.
One interesting 'Harry Potter Vegeta fanfiction' could be about Vegeta somehow ending up at Hogwarts. Maybe he's there to train and he starts challenging the wizards to duels using his Saiyan powers. It would be a wild mix of the magical world of Harry Potter and the powerful universe of Dragon Ball Z where Vegeta comes from.
A 'Harry Potter is Undertaker fanfiction' might involve a magical accident that causes Harry to be connected to the Undertaker in some way. He could start to have visions of the Undertaker's world or even gain the ability to travel between the wizarding world and the world where the Undertaker exists. This could lead to all sorts of adventures as Harry tries to understand and control these new powers while still dealing with his own wizarding responsibilities.
Well, it could be about Zelretch somehow getting involved in the Harry Potter universe. Maybe he uses his powers to change the course of events in Hogwarts, or he could be a hidden figure guiding Harry in a different way than Dumbledore.
A 'Harry Potter is Sasori' fanfiction could start with a strange magical accident that fuses Harry with Sasori's consciousness. Harry starts to see the world from Sasori's perspective and has a new understanding of creation and destruction. He might use his knowledge of both magic and Sasori's puppet - making to create a new form of defense for Hogwarts. He could also face challenges as some in the wizarding world might see his new abilities as dark magic. His friends, like Hermione and Ron, would have to decide whether to support him or be afraid of the changes in him.
It could be about a wizard from the 'Harry Potter' world traveling to the world where 'Senbonzakura' exists and having adventures there, perhaps learning new magic or making new friends.
It could be about Harry Potter somehow getting the powers or the fighting spirit of Kenpachi. Maybe he starts using a zanpakuto - like weapon instead of his wand and battles Death Eaters in a more brutal, Kenpachi - style way.
It could be about Harry using scientific methods to study magic. Maybe he wants to find out the scientific basis behind spells and potions.