It could be a character like Orochimaru who somehow gets access to Gotham's technology and becomes even more powerful. He could be creating new types of jutsu using the advanced tech there and causing chaos in both worlds.
The main antagonist might be a corrupted version of a well - known character from either universe. For example, if a member of the Akatsuki was influenced by Gotham's dark side and became even more ruthless. Or, if one of Batman's rogues gallery somehow got chakra powers and became a major threat to both Naruto and Batgirl.
Maybe a new villain created specifically for this crossover. A criminal mastermind who has learned how to use chakra and combines it with his own devious plans. He could be someone who wants to rule both the Naruto universe and Gotham, and Batgirl and Naruto have to stop him.
It could be Orochimaru from Naruto's world who has found a way to cross into the DC universe. He teams up with some of Batman's villains like the Joker. Orochimaru wants to use the chaos in Gotham to conduct his dark experiments and the Joker just wants to cause mayhem. Batgirl and Naruto would have to deal with Orochimaru's snake - like attacks and the Joker's unpredictable madness.
Madara Uchiha could still be a great antagonist. Even with Itachi and Naruto teaming up, Madara's power and his long - term plans for the ninja world would pose a huge threat. His knowledge of the Sharingan and his connection to the Uchiha clan would make for an interesting conflict against Itachi, while his overall strength would be a challenge for Naruto as well.
A new character could be created. This character could be a powerful slug - human hybrid who has been corrupted by the dark side of both worlds' powers. He wants to rule over both Naruto's world and Slugterra, believing that the two worlds should be united under his rule. He has the ability to use both ninja techniques and slug powers, making him a very difficult opponent to defeat.
Perhaps an ancient and evil ninja from a long - lost clan that was sealed away behind the gate. He resents the modern ninja world and wants to destroy it. He has unique and powerful jutsus that Naruto and his friends have never seen before.
It could be someone like Doctor Octopus who gets his hands on some ninja technology. He could use it to enhance his mechanical arms and also start stealing chakra from people to power his new devices. His scientific mind combined with the newfound ninja powers would make him a formidable foe for Naruto and Spiderman.
It could be Orochimaru. He's always had his eyes on Sasuke and Naruto has also been in his crosshairs. He could return with some new and more powerful jutsu, forcing Naruto and Sasuke to put aside their differences and fight him together. Maybe he has a plan to take over the entire ninja world this time.
It could be a powerful dark spirit from Kagome's world that somehow manages to cross over to the Naruto universe and starts causing chaos. This spirit might be immune to normal ninja attacks and require Kagome's spiritual powers to be defeated.
Maybe another kitsune who has gone bad. In the fanfiction world, there could be a rogue kitsune who sees Naruto as a threat. This antagonist kitsune might be jealous of Naruto's power or his connection to the human world. They could try to undermine Naruto's attempts to protect the village or his friends.
It could be Orochimaru. He might be interested in Ino's mind - control abilities and Naruto's Nine - Tails chakra for his own evil experiments. He could try to capture them both and use their powers to create a new type of jutsu or even a new, more powerful being under his control.
Madara Uchiha would also make a great antagonist. Imagine if he was resurrected and saw Naruto with Samehada as a threat to his plans of creating a new world order. He would use his Sharingan and Rinnegan powers to try and take Samehada from Naruto, believing that with it, he could easily defeat all who oppose him and reshape the ninja world according to his will.