It could be Orochimaru from the Naruto universe. He might have found a way to cross over to the Teen Titans' world and has a new plan to use Naruto's dark side for his own evil purposes.
Another possibility is that Trigon, Raven's father from the Teen Titans, senses Naruto's dark power and wants to use it to break free from his prison or gain more power in the universe. So he manipulates Naruto into doing his bidding and fighting against the Teen Titans.
Maybe a new villain created specifically for this fanfic. A powerful being who feeds on negative energy like Naruto's dark side. This villain could manipulate Naruto to fight the Teen Titans while also draining Naruto's power for its own gain.
Obviously, the evil Naruto himself would be the main antagonist. He's the one causing trouble in the Teen Titans' world, so he takes on that role.
Well, in a 'dark naruto teen titans fanfic', perhaps there's a villain who has the ability to corrupt Naruto and turn him dark. This dark Naruto then teams up with some of the Teen Titans' enemies. The Teen Titans would have to find a way to break the hold on Naruto and defeat their combined foes. They could start by trying to reach out to Naruto's old self through memories or by finding a magical cure.
Perhaps an old member of a forgotten clan who was exiled because of his dark blood. He returns with a vendetta against the village that shunned him. He sees Naruto as a symbol of everything he hates and tries to destroy him. The main antagonist could also manipulate other ninja to turn against Naruto by spreading fear about his dark blood.
The main antagonist could be the Hidden Leaf Village itself. Since Naruto was abandoned by Kushina, the village that failed to take proper care of him could be seen as the enemy. The village elders or those in power who turned a blind eye to his suffering could be the ones he has to oppose.
Well, it could be about Naruto being corrupted by some dark force and then he somehow ends up in the Teen Titans' universe. He might start causing chaos and the Teen Titans have to figure out how to stop him. Maybe he's after some powerful artifact in their world that could enhance his evil powers even more.
Well, obviously Cindastar is one of the main characters. Then there are the classic Teen Titans members like Robin, Starfire, Beast Boy, Cyborg, and Raven. They often interact with Cindastar in different ways, whether it's as friends, teammates, or sometimes even rivals depending on the plot of the fanfic.
Naruto could fit in by sharing his knowledge of the ninja world. This could include things like how to make different types of ninja tools or how to perform certain stealth techniques. In return, the Teen Titans could teach him about teamwork in a different way than he's used to in Konoha. His unique abilities like summoning animals could also be a huge asset when they are up against tough enemies.
It could be an unknown ninja who has been secretly studying dark energon for years. He wants to use it to take over the ninja world, believing that the power of dark energon will make him invincible.
It could be a rogue Dark Jedi who has escaped from his own Order and sees the Naruto world as a new place to spread chaos. He might be seeking powerful chakra sources to fuel his Dark Side powers.
It could be Sasuke. If Sasuke continues to pursue his own vendetta in a more extreme way and Naruto's attempts to bring him back fail, Sasuke could become the main antagonist. Their rivalry could turn into a full - blown battle for power and ideology, with Naruto being pushed to his limits and potentially turning dark in the process of trying to stop Sasuke.