Who could be the main antagonist in a 'Velvet Scarlentina x Naruto' fanfic?

One idea could be that Velvet Scarlentina, with her unique abilities (assuming she has some from her original context), somehow gets transported to the Naruto world. She could meet Naruto and his friends and join them on a mission. Maybe she has a special power related to healing that comes in handy during a tough battle.

It could be Orochimaru. He's always been interested in unique powers and abilities. The idea of X - 23's mutant abilities and Naruto's jinchuriki powers would be very appealing to him. He might try to capture them both to use in his experiments to gain ultimate power.

Maybe an evil sorcerer from a third, unknown dimension who sees the potential in both Naruto's chakra and Pneuma's power. He plans to steal their powers to become invincible. He starts by creating unrest in both their worlds, pitting their friends and allies against them. Naruto and Pneuma must figure out his true identity and stop him before he succeeds in his devious plan.

It could be an evil ninja who has somehow gained access to the technology from Neo's world and wants to use it to take over the Naruto world. He might be able to create clones using digital technology, making him a very difficult enemy to defeat.

It could be a rogue member of Kara who has their own agenda and doesn't agree with any form of alliance with Naruto or the Hidden Leaf. This character might try to undermine any cooperation between Naruto and the rest of Kara.

Madara Uchiha could be a great antagonist. He could have somehow gained the power to cross into the Bleach universe and is causing trouble there. Yoruichi and Naruto would have to put aside their differences and combine their skills to stop him. Madara's powerful jutsu and his desire for power would make him a formidable foe for the duo.

It could be an organization of dark summoners. They believe that by sacrificing Matatabi, they can gain ultimate power. They send out their best agents to hunt down Matatabi and Naruto. These agents have the ability to manipulate dark chakra, which is harmful to both Naruto and Matatabi. The organization's leader is a powerful sorcerer who has been planning this for years.

It could be a rogue ninja who has a vendetta against Naruto and sees Sister Lemon as an easy target to get to Naruto. This ninja might have some special powers that make him a formidable foe, like the ability to control shadows in a more sinister way than Naruto's shadow clone jutsu.

A powerful demon or dark spirit could be the main antagonist. This entity could be attracted to the combined power of Naruto and fem Apollo. It might seek to drain their powers to become even more powerful itself. For example, it could be a demon from Naruto's world that has somehow crossed over into Apollo's realm and is causing chaos there, targeting both Naruto and fem Apollo because they pose a threat to its plans.

It could be a powerful rogue ninja who has a vendetta against Kakashi and sees his daughter as a way to get revenge. This antagonist might try to use her as a bait to lure Kakashi or Naruto into a trap.