There could be a new dark wizard or witch emerging. This new villain could be jealous of the Potter family's status and power. They might target the third child as a way to get back at Harry and Ginny, thinking that by harming their child, they can break the family and undermine the stability that the Potters represent in the wizarding community.
The remnants of Voldemort's followers could still be a threat. They might see the third child as a symbol of the new hope in the wizarding world that they want to destroy. Maybe they believe that the child has some sort of power that could ultimately be used to completely wipe out their dark ideology, so they target the child.
A new dark wizard could rise. This wizard might be jealous of the fame and power that Harry has, and seeing his daughter as an extension of that, decides to target her. Maybe this new dark wizard is seeking to gain control over the wizarding world and sees her as a key obstacle in his path.
There could be a new dark wizard or witch who is jealous of the Potter family's legacy. They might target Harry Potter's daughter to try and bring down the family name and prove their own power.
One possible storyline could be that the third child discovers they have a unique magical ability that neither Harry nor Ginny have. For example, they might be able to communicate with magical creatures in a way no one else can. This could lead to all sorts of adventures as they explore this new power and how it can be used for good in the wizarding world.
The Death Eaters could still be a major threat. Even with Voldemort gone, some of them might be trying to revive the dark ways and see the wizard king as a big obstacle in their path.
The obvious enemies could be the Ministry of Magic. If Harry is stealing things, even if for good reasons, they would see him as a criminal. The Aurors would be sent after him. Another potential enemy could be some of the dark wizards who might want the same items that Harry is after. They would not want Harry to get in their way and would try to stop him or even capture him to get information on the whereabouts of the valuable items.
Another potential enemy could be a group of pure - blood supremacists who don't like the idea of the White Knight, who might be a symbol of change and equality in the wizarding world. They would try to undermine the White Knight's efforts to make the wizarding world more inclusive.
The First Order could still be a threat. They might see her as a symbol of the Resistance and try to eliminate her. Or it could be a new Sith cult that has risen in the ashes of the old Sith order.
The Ministry of Magic would likely be an enemy. They are all about maintaining order and the Demon Prince's actions might be seen as a threat to that order.
It could be someone completely new, an original character created for the fanfiction. Maybe a mysterious wizard who had a brief encounter with Ginny.
Voldemort or his remaining Death Eaters could be the main enemies. They would see Harry's new blade master abilities as a threat. Another potential enemy could be a dark wizard from a foreign land who is jealous of Harry's power.