It could be an ancient dark side entity that has been awakened. This entity is drawn to Ahsoka Tano's strong connection to the Force. It wants to corrupt her or destroy her to spread its dark influence across the galaxy.
Well, one idea could be to focus on Ahsoka Tano's early days as a Padawan. Maybe she has to face a new threat on a remote planet while trying to prove herself to her Master. She could discover an ancient Sith artifact that puts her in danger but also gives her a chance to show her skills and wisdom.
Darth Maul could be a great antagonist. His double - bladed lightsaber would be a formidable weapon against Naruto. Maul's acrobatic fighting style and his Sith powers would pose a huge challenge to Naruto's ninja skills and new - found Jedi powers.
Maybe Darth Vader. If the story is set at a time when Vader is at the height of his power, he could be the main obstacle in Mace Windu's path of revenge. Vader's loyalty to the Emperor and his own dark side powers would make him a formidable foe for Windu.
It could be a rogue Sith Lord who has broken away from the traditional Sith teachings and is causing destruction in his own way. He might not be interested in the age - old battle between the Sith and the Jedi but just wants to spread chaos for his own reasons.
Perhaps an ancient evil force that Darth Vectivus accidentally awakens. This force is much more powerful than him and starts to control him, making him do things he doesn't really want to. Now Darth Vectivus has to find a way to break free from this new master while also dealing with the rest of the galaxy that sees him as the bad guy.
Darth Vader could be a great main antagonist. His power in the Force and his imposing presence would be a huge threat to The Flash. He could try to capture The Flash to study his speed abilities for the Empire.
Kylo Ren could still be the main antagonist even if Rey turns to the dark side. He might see her as a threat to his own power within the dark side hierarchy. Maybe he wants to keep her in check or even destroy her to maintain his dominance.
A new, unknown dark side entity could be the main antagonist. Maybe Revan's time travel creates a rift in the Force that awakens a powerful being that feeds on chaos and destruction. This being would oppose Revan because his actions of trying to fix the timeline would go against the chaos that the entity thrives on.
It's hard to say for sure. If it's related to Ahsoka Tano, her story has been expanding in recent years with new shows and content, which could be considered new additions in a way. But if it's something else entirely, without more context, it's difficult to determine if it's a new addition.
Anakin Skywalker is a very likely character as he was Ahsoka's master. Also, characters like Obi - Wan Kenobi might make an appearance since they are all part of the same Star Wars universe. There could be some of Ahsoka's old friends from the Clone Wars era.