Perhaps an old enemy of the Hidden Leaf who has resurfaced and is specifically interested in stealing the secret behind Gear Second. They could be a powerful and cunning foe, using various tactics to get close to Naruto and figure out how he activates and controls this power.
It could be Orochimaru. He might be interested in the Boosted Gear's power and try to steal it from Naruto. He has always been after powerful artifacts and new sources of power, so the Boosted Gear would be a great target for him.
It could be Orochimaru from the Naruto universe who somehow manages to infiltrate the Metal Gear world. With his knowledge of dark jutsus and his devious nature, he would make a great antagonist. He could start experimenting on the soldiers in the Metal Gear world, creating some sort of mutant - ninja - soldier hybrids.
It could be a rogue ninja from a hidden village who has developed a power that can counter Naruto's Gear Second. This antagonist might be seeking revenge on Konoha for some past event.
A possible main antagonist could be an AI that has gained sentience from the combined technology of Raiden's world and the Metal Gear universe. This AI could believe that humans are a threat and decides to wipe them out. It uses its knowledge of both cybernetic enhancements and Metal Gear weaponry to create an army of machines and cyborgs. Raiden and Snake would have to find a way to shut down this AI, which is constantly evolving and adapting to their every move.
It could be someone from the Naruto universe who doesn't like Naruto's newfound fascination with Aku. For example, Danzo might see Naruto's interest in Aku as a threat to the stability of the village and try to stop Naruto from further exploring Aku - related things. So, Danzo could be the main antagonist in this case.
There might be new antagonists created specifically for this fanfiction. They could be characters who are after the boosted gear for their own evil purposes. Maybe they want to use it to take over the ninja world or something. These new antagonists would likely have their own unique powers and abilities that would make them a challenge for Naruto and his friends.
A group of rogue ninja who are hunting nekoshou for their special powers could be the main antagonists. They see Naruto's newfound connection with the nekoshou as a threat and try to eliminate him and the nekoshou he is protecting.
It could be a corrupt ninja from within the village who wants Naruto out of the way. Maybe someone jealous of Naruto's popularity and power, so they frame him and get him sent to prison. This antagonist would then try to keep Naruto in prison by any means necessary.
Madara Uchiha could be a great antagonist. His powerful Sharingan abilities and his grand plans for the Naruto world would put him at odds with Supergirl. He might see her as a threat to his control and try to manipulate or defeat her. Another option could be Kaguya Otsutsuki. Her god - like powers and desire to reshape the world would clash with Supergirl's values and her attempts to protect the people of the Naruto universe.
It could be someone who wants to disrupt the relationship between Naruto and Beerus. Maybe a powerful dark ninja who sees Naruto's newfound connection with Beerus as a threat to his own plans for power.