Perhaps an organization that sees Naruto as a threat because of his 'son of scar' status. They might fear that he will grow up to be very powerful and oppose their plans. So, they try to eliminate him before he becomes too strong. They could send out assassins or set up various traps for Naruto and his friends.
Another possibility is a jealous ninja who has some grudge against Naruto's family. He might see Naruto as an easy target because of the rumors and superstitions surrounding the scar. This antagonist could try to frame Naruto for crimes he didn't commit, in order to turn the village against him and isolate him from his support system.
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.
The plot might revolve around Naruto being ostracized because of his relation to the 'son of scar' title. He then has to prove himself not only as a ninja but also to overcome the stigma associated with this identity. This could lead to many adventures where he battles against those who misjudge him and in the process, uncovers the truth behind the scar and his heritage.
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.
It could be Orochimaru. He's always after power and seeing Naruto with Son Goku's power would make him a prime target. He might try to extract the power from Naruto for his own evil experiments.
It could be someone from the Naruto universe who doesn't like Naruto's newfound fascination with Aku. For example, Danzo might see Naruto's interest in Aku as a threat to the stability of the village and try to stop Naruto from further exploring Aku - related things. So, Danzo could be the main antagonist in this case.
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.
A group of rogue ninja who are hunting nekoshou for their special powers could be the main antagonists. They see Naruto's newfound connection with the nekoshou as a threat and try to eliminate him and the nekoshou he is protecting.
It could be a corrupt ninja from within the village who wants Naruto out of the way. Maybe someone jealous of Naruto's popularity and power, so they frame him and get him sent to prison. This antagonist would then try to keep Naruto in prison by any means necessary.
Madara Uchiha could be a great antagonist. His powerful Sharingan abilities and his grand plans for the Naruto world would put him at odds with Supergirl. He might see her as a threat to his control and try to manipulate or defeat her. Another option could be Kaguya Otsutsuki. Her god - like powers and desire to reshape the world would clash with Supergirl's values and her attempts to protect the people of the Naruto universe.
It could be someone who wants to disrupt the relationship between Naruto and Beerus. Maybe a powerful dark ninja who sees Naruto's newfound connection with Beerus as a threat to his own plans for power.