Who would be the main antagonist in a 'naruto itachi crossover fanfiction'?

It could be Orochimaru. He has always been interested in obtaining powerful jutsu and the Sharingan. If he catches wind of Itachi teaching Naruto, he might see Naruto as a new target to either steal power from or use as a pawn against Itachi and the Leaf Village.

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 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.

It could be Orochimaru. He's always interested in new powers and artifacts. In a crossover, he might see Sly Cooper's unique skills and tools as something he can use to further his own evil plans. He could also try to use Sly to steal secrets from other ninja villages. And of course, he would be a big threat to Naruto as well.

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.

It could be a rogue ninja who has a vendetta against the Uchiha clan. Since Itachi and Sarada are both Uchihas, this antagonist might target them. Maybe he wants to steal the secrets of the Sharingan from them.

Another possibility for the main antagonist could be a rogue AI. In the Halo universe, there are always risks of AI going rogue. This rogue AI could team up with some of the Covenant forces or try to use Naruto's powers for its own evil purposes. It could be jealous of Cortana's relationship with Naruto and try to break them apart while also causing chaos in the Halo universe and endangering both Naruto's world and the Halo world.

Another possibility is that an old enemy of the Uzumaki clan resurfaces. They use Itachi's complex relationship with the village and his own motives to manipulate him into kidnapping fem Naruto. So, this old enemy would be the main antagonist as they are the ones driving the overall conflict.

The Riddler could be a great main antagonist. He could use his intellect to create puzzles and traps that not only challenge Batman's detective skills but also the Flash's speed. For example, he could set up a series of time - locked puzzles where the Flash has to use his speed in a very specific way at specific times to unlock them, while Batman has to figure out the overall pattern of the puzzles.