He might meet descendants of the Valyrian people. These could be people who have managed to survive the cataclysm that befell Valyria and have unique knowledge about the place and its history.
Another popular character could be Tyrion Lannister. His intelligence and cunning make him an interesting addition to Valyria - themed fanfictions. He could be on a journey to Valyria to seek knowledge or perhaps to find a solution to the political unrest in Westeros. His unique perspective would add a lot of depth to the stories set in the Valyrian context.
Dr Doom, in the comics, is Victor von Doom. He's a scientific genius with a penchant for sorcery and a desire for world domination. His complex personality and advanced technology make him a formidable foe.
Jon Snow could discover ancient Valyrian magic. Since Valyria was a center of magic before its fall, he may stumble upon spells or artifacts that could change his fate or give him new powers. He might also meet other characters who have a connection to Valyria's past, and together they explore the ruins and uncover long - forgotten secrets about the once - great civilization.
The Death Eaters could be one group of antagonists. They might see Harry's newfound power and position as the King of Valyria as a threat to their plans for wizarding world domination. So they try to stop him from fully realizing his potential in Valyria and bringing back any new magic or alliances that could defeat them.
There could be characters like Reed Richards. He is often in conflict with Doctor Doom. Richards represents the scientific and heroic side, which contrasts with Doom's more megalomaniac tendencies.
I don't know the author of 'king of doom novel' as there isn't enough information given. It could be an indie author or from a relatively unknown publisher.
Victor Von Doom is one of the most notorious antagonists in comics. His ego and ambition lead him to take on the heroes, using his technological and magical prowess. Despite his villainous nature, he has a certain charm and depth to his character.