Who could be the main antagonist in a 'naruto x prototype crossover fanfiction'?

It could be an old enemy of Kushina's who was thought to be dead but has now returned. Maybe someone from her days in the Uzumaki clan who has a vendetta against her and now sees Naruto as an obstacle too.

A good antagonist could be a character from the 'Resident Evil' universe, like Albert Wesker. His superhuman abilities and evil plans for global domination would pose a huge threat to Alex Rider. Alex would have to use his wits and skills to stop Wesker's bio - terrorist plans.

Maybe the main antagonist is an artificial intelligence on the Clockwork Planet that views Naruto and his chakra abilities as a threat to its control over the planet. It starts to send mechanical beasts to Naruto's world to capture him and study his powers. This AI is highly intelligent and has a vast network of clockwork devices at its disposal, making it a formidable foe for Naruto and his friends.

It could be a powerful demon lord who wants to conquer both the Naruto universe and the Devil May Cry world. This demon lord might have the ability to manipulate chakra and also use demonic powers, making it a formidable foe for both Naruto and Dante.

Maybe a corrupted version of one of the Sailor Scouts. She could be jealous of Naruto's attention towards the other Scouts in the harem. So, she makes a deal with a dark force from Naruto's world, like a powerful Orochimaru - like character. This combined power of dark jutsu and a twisted Sailor power would create a very interesting and complex main antagonist for this crossover fanfiction.

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 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.

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.