It could be Akuma. He is extremely powerful in the Street Fighter universe with his dark Hadou powers. In a combined universe, his ability to manipulate dark energy could be enhanced by the magic in the Harry Potter world. He could potentially learn spells and use them in combination with his fighting skills, making him an almost unstoppable force.
One way is through the power systems. The chakra in Naruto and the energy used in Street Fighter can be merged. For instance, a character could use chakra to enhance the power of a Street Fighter move like Chun - Li's Spinning Bird Kick.
Thor could also be very powerful. He has his Asgardian strength and his hammer (or Stormbreaker). If he were to also gain the ability to use magic like the wizards in Harry Potter, he could be near - invincible. He could call lightning while also casting spells. And his knowledge of different realms could give him an edge in any battles that span across the combined universes.
It could be Albus Dumbledore. He has great magical knowledge and power in the 'Harry Potter' universe. When combined with the 'Fate Stay Night' elements, he might be able to outwit even the most powerful Servants. His ability to plan and his understanding of magic could give him an edge in any conflict.
It could be Naruto himself. His ability to use massive amounts of chakra, create shadow clones, and use powerful jutsu like the Rasengan and the Sage Mode would make him a formidable opponent. He could adapt his ninja skills to the Street Fighter fighting style easily.
Well, it could be that the magic from Harry Potter's world and Android 18's advanced technology somehow interact. Maybe the spells in Harry Potter are used to enhance or modify Android 18's abilities. For example, a spell could be cast on her to make her energy blasts more powerful or her speed even faster.
Probably a combination of Deku and Ken. Deku's One For All power gives him incredible strength and speed, and Ken's Shoryuken and other moves would add a whole new level of combat ability. Deku could use Ken's fighting style to better control his power, and Ken could benefit from Deku's raw power. They could take on any enemy in the combined universes with ease.
Well, one way is to have the magic from Harry Potter interact with the physical threats of the Tremors universe. For instance, the protective spells around Hogwarts could be modified to keep out the Tremors creatures. Or the Portkeys could be used to transport characters to areas affected by Tremors.
In a possible plot, the Ministry of Magic decides to introduce a new form of combat training based on Street Fighter to better prepare wizards for dark threats. Harry Potter is skeptical at first but then realizes the potential. As he trains, he uncovers a conspiracy within the Ministry that is using this new training method for their own evil purposes. He then has to use his skills from both magic and Street Fighter to expose the conspiracy and save the wizarding world.
One great fem Harry Potter Street Fighter fanfiction is 'The Wizard in the Ring'. It combines the magical world of Harry Potter with the intense fighting style of Street Fighter. In this story, Hermione, as a female version of Harry Potter, discovers she has some unique fighting abilities that are more in line with Street Fighter moves. It's really exciting to see how the author blends the two universes.
One way to combine the superhero and Harry Potter universes in fanfic is through a magical accident. For example, a spell gone wrong could transport Harry Potter and his friends to a world full of superheroes. There, they might learn new powers or use their magic in unexpected ways to help the superheroes fight villains.