Well, it's possible. If we consider the fact that the ancient Sith were the originators of many Dark Side techniques. They might have had a purer, more raw form of the Dark Side power. Sidious, while extremely powerful, was a product of a more refined and political Sith system. The ancient Sith's unpredictable and wild use of the Dark Side could give them an edge in a fictional battle.
By using forgotten Dark Side rituals. Ancient Sith might have known rituals that could directly counter Sidious' powers. For example, a ritual to disrupt his connection to the Force momentarily.
Well, one way could be that the ancient Sith have a different code or view from Sidious. Maybe they see Sidious as a threat to the balance of the Force in a way that aligns with the Jedi's view for a short time. They might also have a vendetta against Sidious for some power - grabbing within the Sith ranks that the Jedi could use to their advantage.
Some Jedi would be extremely suspicious. They've been taught all their lives that the Sith are the enemy. They might think it's a trap by the Sith. For example, young padawans who haven't had much exposure to the complex politics of the galaxy would be very wary.
Maybe Sidious is trying to say that it's not just a simple fictional story. It could imply that the events related to him are real in the Star Wars universe in a sense. Like his rise to power, his manipulations, are all very real threats and not something to be dismissed as a made - up tale.
He could inherit the power of Force lightning. Darth Sidious was known for his powerful Force lightning attacks. In the Naruto universe, this could translate to some sort of unique chakra - based lightning attack that is far more powerful than the normal lightning - style jutsu. Also, Naruto might inherit some telekinetic abilities, being able to move objects with his mind or chakra, similar to how Sidious could use the Force for telekinesis.
Well, a great fanfiction idea could be that Darth Sidious discovers the existence of the wizarding world through some dark magic connection. He decides to conquer it, thinking it would be easy prey. But he underestimates the power of the Hogwarts students and their loyalty to each other. Harry Potter would be at the forefront of the resistance, using his skills and the support of his friends like Hermione and Ron.
One idea could be a crossover where Darth Sidious discovers the world of Naruto and tries to manipulate the ninja politics for his own gain. He might see the chakra system as a new source of power to exploit.
Well, if Naruto were the grandson of Darth Sidious in this fanfiction, it would likely bring a dark and different dynamic to the Naruto universe. Naruto's typically positive and bright nature might be conflicted with the Sith influence from Darth Sidious. His values of friendship and hard work could be challenged as the Sith ways are more about power and domination. It could also change the power balance in the Hidden Leaf Village as Naruto might have access to different, perhaps more sinister powers, which would make his fellow ninjas view him differently.
A fanfiction could explore the idea of Darth Sidious being somehow trapped in the Harry Potter universe. He'd be completely out of his element, yet still a formidable threat. Harry Potter, with his knowledge of the wizarding world and his courage, would be the one to stand up to Sidious. This could lead to a unique mentorship situation where Dumbledore tries to understand the nature of the Force from Sidious in order to find a way to defeat him, while also teaching Harry how to resist the Sith's temptations.