It could be a Decepticon who sees the 'Son of the Stars' as a threat to their plans for domination. Maybe a powerful Decepticon leader who fears the special powers that the 'Son of the Stars' might possess.
Perhaps an ancient evil that has been awakened by the presence of the 'Son of the Stars'. This entity could be from a time long before the Transformers and wants to use the power of the stars for its own dark purposes. It would be a formidable foe as it has knowledge and powers that the Transformers are not familiar with.
There might be a rogue Autobot who has become jealous of the 'Son of the Stars'. This Autobot could feel that the 'Son of the Stars' is getting all the attention and resources, and so decides to turn against his own kind and become an antagonist. He would know the weaknesses of the Autobots and use that knowledge against them.
It could be someone who wants to use Naruto's connection to the scar for their own evil purposes. Maybe a rogue ninja who believes that the scar holds some kind of dark power that can be harnessed.
It could be a rogue ninja who has a vendetta against Naruto. This ninja might see Naruto's son as a way to get back at Naruto for past defeats or slights.
One key theme could be identity. In a Transformers fanfiction with 'Son of the Stars', the character might struggle with his place in the universe, being related to the stars in some way. Another theme could be power. With a connection to the stars, he might have some special powers that he has to learn to control. And there could also be a theme of loyalty, as he might have to decide which side to be on in the Transformers' ongoing battles.
A powerful dark wizard who has a grudge against those with fire - related magic could be the main antagonist. He might see Harry's newfound 'Son of Fire' powers as a threat to his own plans for domination.
A new dark wizard could be the main antagonist. Maybe someone who was inspired by Voldemort's ideology but has new and more devious ways of causing chaos in the wizarding world. This antagonist could target Harry Potter's son to prove himself as a more powerful and feared dark wizard.
It could be someone who wants to use Naruto's unique heritage against him. Maybe a villain from the Naruto universe who sees Naruto's connection to Harley Quinn as a weakness. For example, Orochimaru might want to capture Naruto to study his strange combination of powers.
Perhaps an ancient and evil creature that has a grudge against Oin. Since Percy is Oin's son, this creature targets him. It could be a monster that was once defeated by Oin and now wants revenge by going after his son.
Another option could be a corrupted demigod. Maybe a child of a minor Egyptian god who is jealous of Percy's status as Son of Sobek. This demigod could form an alliance with some dark forces and try to undermine Percy at every turn, creating all sorts of challenges for him.
It could be an ancient evil force that wants to use the Dark Master Son's power for its own dark purposes. This force might see Spyro as an obstacle in its plan to dominate the world.
Perhaps an ancient sea god who was once an enemy of Pontus. This god could see Percy as a threat because of his heritage. He would use his own minions, like sea serpents and murlocs, to attack Percy and those he cares about. He might also try to corrupt Percy, tempting him with power to turn against his friends.