Most likely, Voldemort would remain the main villain. Without Harry to stop him, he would be free to carry out his evil plans unopposed.
It could also be that some of the more power - hungry Death Eaters would try to take over from Voldemort and become the new main villains. They might see his victory as an opportunity to gain more power for themselves and start causing chaos in the wizarding world.
One possible plot development could be that his friends are left to pick up the pieces. They might struggle with grief and guilt, thinking they could have saved him. Another could be that Voldemort finally takes over completely without Harry to oppose him, leading to a dark reign over the wizarding world.
It could be a new dark wizard who has infiltrated Hogwarts under the guise of a new teacher. This wizard might have a vendetta against Harry specifically and sets up elaborate traps to kill him.
Dumbledore would feel a great sense of loss and guilt. He had always seen Harry as the key to defeating Voldemort. His reaction could be to go into seclusion for a while to think about what went wrong. The other Hogwarts professors would be in shock. Hagrid would be heartbroken and might even leave Hogwarts in his grief. And the Death Eaters would be jubilant, but also a bit wary as they know that there could be unforeseen consequences to Harry's death.
One possible plot development could be that Voldemort fully regains his power without Harry as an obstacle. The wizarding world would be plunged into complete darkness. His friends, like Hermione and Ron, might go into a deep state of despair and then form a resistance group. They would try to find other ways to defeat Voldemort, perhaps by seeking out more powerful magical artifacts or uniting with some previously neutral magical creatures.
Voldemort would surely be a main character. With Harry dead as a baby, Voldemort's actions and how he further consolidates his power would be a major part of the story.
The Basilisk is an obvious main antagonist. It is a huge and powerful magical creature that has the ability to kill with a single glance. In the original story, Harry barely manages to defeat it. In a fanfiction where Harry dies, it could be that the Basilisk is just too strong for him to overcome.
Well, in this kind of fanfic, Harry could be neglected by Hogwarts staff. For example, teachers may be too preoccupied with other matters, such as political unrest in the wizarding world. Harry, feeling alone and abandoned, might venture into dangerous areas like the Forbidden Forest more often. He could encounter a powerful and malevolent creature that he's not ready to face, and ultimately die because there was no one to rescue him. Also, the Sorting Hat could potentially mis - sort him into a house where he doesn't fit in at all, and the neglect from his housemates could contribute to his downfall.
A new student at Hogwarts could be the main character. They might be a bit of an outcast and find solace in the graveyard. There, they stumble upon a mystery that no one else has noticed before, and it becomes their mission to solve it. This would be a great way to introduce a fresh perspective to the Harry Potter fanfic world.
It could be Voldemort or his remaining Death Eaters. Since they have always been the main antagonists in the Harry Potter universe, in this fanfiction they might still be the ones causing the danger that leads to Harry's sacrifice for Ginny.
It could be an unknown, ancient evil that has been awakened. This villain might have powers far beyond what Naruto and Sasuke have faced before. Maybe it's an entity from a forgotten era that was sealed away long ago and now has broken free.