In the wizarding world, there might be some jealous wizards who don't like Harry's unique combination of powers. They could team up with dark forces, maybe some Death Eaters who are still lurking around, and try to bring Harry down. They might be envious of the attention Harry gets or fear his new - found powers will disrupt the traditional ways of magic.
Well, it could be about Harry having the speed force abilities from Barry Allen. Maybe he uses those powers to quickly escape from Voldemort's attacks or to quickly find Horcruxes. He could also use his speed to outmaneuver his enemies at Hogwarts like Malfoy and his gang.
Well, aside from Voldemort, there might be corrupt elements within the assassin organization that raised Harry. They could see his magic as a threat to their power and secrecy. Also, there could be some dark wizards who team up with the assassin enemies of Harry. These villains would be more complex as they combine the danger of dark magic with the stealth and combat skills of assassins. And there could be a new character, perhaps an assassin who was once a friend but turns bad due to jealousy or a thirst for power, creating a more personal and dangerous threat to Harry.
Voldemort would still be a likely villain. He is the main antagonist in the original series and his threat to Harry is always present. Even if Harry is trying to save himself, Voldemort's power and his desire to kill Harry would make him a formidable foe.
Yes, there are quite a few out there. You can start by looking on popular fanfiction websites like Fanfiction.net or Archive of Our Own. Just search for 'Harry Potter Barry Allen' and you'll find a variety of stories that blend the two universes in interesting ways.
One great Barry Allen and Harry Potter fanfiction is 'The Flash in Hogwarts'. It's really interesting as it combines the speedster powers of Barry Allen with the magical world of Harry Potter. The author does a great job of making the two universes blend well, like when Barry has to navigate the moving staircases at Hogwarts while still being able to use his super speed in creative ways.
One potential villain could be Voldemort from the Harry Potter world. He might see Harry's new alliance with Erza as a threat and try to destroy it. Also, there could be villains from Erza's world who are jealous of the power that Harry and Erza together possess.
The combination of two very different universes. Harry Potter has magic, wizards, and a complex magical society, while Barry Allen is a superhero with super - speed. Merging these two concepts can lead to really unique and unexpected storylines.
Hermione would definitely be one of his allies. She is smart and would be able to help him research about his new identity and how to control his powers. For example, she could find ancient texts in the Hogwarts library that relate to both the wizarding world and the Greek gods.
His friends from Hogwarts could still be his allies. Hermione would use her intelligence to help him understand his new situation. Ron would be there for moral support. Additionally, any friends or allies that Rias has in her world could also become Harry's allies. They might be magical creatures or other powerful mages.
Hermione would definitely be an ally. She's smart and would be able to help Harry research about his 'son of Mars' heritage, finding ancient spells or texts that could explain it further.