It could be an OC (Original Character) who is a half - human, half - demon. This character has been shunned by society and thus turns to a life of crime, becoming the gangster queen. She would have her own set of powers and a complex backstory that ties into the Naruto world. Maybe she has a connection to the Akatsuki in some way.
Maybe the main character is a foreign ninja who comes to the Naruto world with her own set of rules and a gangster - like attitude. She could be from a land where the ninja culture is different, and she tries to impose her ways in the Hidden Leaf or other villages. This would create a lot of conflict and interesting interactions with the existing Naruto characters.
Well, it could be about a female character who is like a gangster queen in the Naruto world. Maybe she's a powerful kunoichi who rules an underground organization. She could be involved in illegal trading of ninja tools or secrets. And she might cross paths with Naruto and his friends, either as an enemy or an unexpected ally.
Another option could be Orochimaru. He has his own motives and is always looking for power. He could be the head of a powerful gang in the criminal underworld of the Naruto world. He would constantly be trying to capture Naruto or his friends to use them for his experiments or to gain more power over the ninja world.
It could be a self - made queen from a far - off land. Maybe she is a powerful magic user who has united her land through her own strength and wisdom. When she meets Naruto, she is intrigued by his unique ninja abilities and they fall in love.
It could be a powerful dark wizard or sorcerer who wants to control both Naruto's world and the Spider Queen's domain. This antagonist might be after the unique powers that Naruto and the Spider Queen possess, like Naruto's chakra and the Spider Queen's web - based magic. He could try to manipulate them against each other to achieve his goals.
It could be a powerful demon who wants to control the Succubus Queen and use her powers for evil. Since Naruto is with the Queen, he becomes a target too.
It could still be Naruto himself, just with a focus on a different, smaller part of his life or personality.
It could also be a young apprentice of the shopkeeper. This character could be learning the ropes of running the shop while also having dreams of becoming a great ninja. They would be in a position to interact with both the regular customers of the shop and the more famous ninjas, which could lead to all sorts of interesting storylines.
It could be Megaman himself. He could be transported to the Naruto world and have to adapt to the new environment while using his skills to make a difference. His sense of justice would fit well in the Naruto universe.
It could be an original character created specifically for the fanfiction. Maybe a rogue ninja with a vendetta against the Hidden Leaf Village.
Naruto himself could be the main character. Imagine Naruto getting his hands on a special pistol - like weapon and having to learn how to use it while still relying on his normal ninja skills. It would be a new challenge for him and could lead to some great character development as he tries to master this different form of combat.