There could be a young vampire who looks up to the forgotten overlord. This young vampire was born after the overlord was forgotten but has heard tales of his great power. He decides to find the overlord and help him rise again. Also, a human psychic might be a main character. The psychic can sense the presence of the vampire overlord and is drawn to him, either out of curiosity or a sense of a greater purpose.
One possible main character could be a vampire hunter. This hunter might initially be after the forgotten overlord but then realizes that there are greater threats in the vampire world and teams up with the overlord. Another character could be a witch who has knowledge about the spell that made the overlord forgotten and helps him to break it.
The plot could involve the vampire overlord being awakened from a long slumber. Maybe he discovers a world that has changed a great deal since he was last in power. He might have to deal with new enemies or form alliances with unexpected characters to regain his former status.
One could be the Overlord himself. He might interact with the child in various ways, either as a mentor, an adversary, or an unexpected ally. His vast power and knowledge of the world would make for interesting dynamics with the child.
Ainz Ooal Gown is one of the main characters. He is a very powerful figure in the story and his actions and decisions often have a great impact on the overall plot.
Another character that could be important is Dumbledore, even if he's dead in the original series. In fanfiction, he could appear as a ghostly mentor to Harry, guiding him on his path as the Overlord. And then there are the villains or antagonists, like Voldemort's followers who might oppose Harry's new regime.
Typically, in Overlord vampire fanfiction, there are often elements of the vampire's immortality and thirst for blood combined with the power - hungry and hierarchical world of Overlord. The vampires usually have enhanced physical abilities like super strength and speed.
Albedo is also a very important character. She is deeply in love with Ainz and is one of the guardians of the Great Tomb of Nazarick. She is extremely powerful and plays a crucial role in Ainz's plans and the protection of Nazarick.
The main character is Momonga. He is a very powerful being in the story. His transformation and growth throughout the novels are central to the plot.
Obviously, the demon overlord could be the main antagonist. He might have his own motives like ruling the ninja world or obtaining some powerful jutsu that Naruto and his friends are trying to protect.
A young prodigy who has been underestimated all his life. Fed up with being pushed around, he uses his intelligence and unique abilities to rise to power and become the world overlord.
It could be a young and ambitious wizard who has come to the town to assert his magical dominance.