There could be a dark spirit or entity that has been awakened by Harry's arrival in New Orleans. This entity was once sealed away but Harry's magic has somehow loosened its bonds. It then starts causing chaos in the city, attacking both muggles and wizards alike. Harry has to figure out how to stop this entity, but first he has to deal with the fact that the local magical community blames him for the entity's awakening.
In this fanfiction, Harry could be on a quest related to some ancient magic that is tied to New Orleans. He may end up getting involved with the local magical community's power struggles. For example, there could be a fight over a powerful voodoo talisman, and Harry's skills from his Hogwarts days come in handy to help resolve the situation. Also, he might start to learn about the unique magical creatures that are native to the area.
It could be Voldemort. Even though he's from the Harry Potter universe, if he finds out that Harry is hiding in Forks, he could send his Death Eaters there. This would not only put Harry in danger but also the Cullens and the people of Forks as the Death Eaters might start causing chaos.
It could still be Voldemort, but in a different form. Maybe he has a new power or a different plan this time. Since it's a new chance, he might have found a way to come back even stronger and more devious than before.
Maybe a group of pure - blood supremacists who didn't like the changes that Harry brought to the wizarding world after the war. They believe that the old ways of pure - blood dominance should be restored and will stop at nothing to oppose Harry's new beginning, which includes more equality among all wizards and witches.
There could be a local artist. This person could introduce Bella to the art community in New Orleans and help her see the city from an artistic perspective.
Perhaps a long - lost relative of the Potter family who feels they were unjustly excluded from inheriting the manor. This relative could be jealous of Harry's status and the wealth associated with the manor. They might use dark magic to try and claim what they think is rightfully theirs, leading to a battle of magic and loyalty within the Potter family and among Harry's friends.
It could be a new dark wizard who has emerged in Harry's new life. This wizard might be jealous of Harry's fame and want to take him down.
Scarecrow could also be a main antagonist. His fear - inducing toxins could be a real problem for Harry. Harry would have to use his magic to counter the effects of the fear toxin and also figure out a way to stop Scarecrow from spreading it all over Gotham.
A new dark wizard or witch could be created specifically for this fanfiction. This character could be interested in using the Huntsman's skills for their own evil purposes. If the Huntsman refuses, they become enemies. This antagonist could have powers related to dark forest magic, something that the Huntsman is familiar with but in a more malevolent form.
Voldemort could still be the main antagonist. Even if Demetri has some sort of connection or change in the fanfiction, Voldemort's overarching evil plans would still be a major threat to both Harry and Demetri, especially if Demetri tries to deviate from the Death Eaters' path.