Maybe a tech - based villain who can control and disrupt Metalman's systems. This villain could see Metalman as a threat to his plan of world domination through technology. Iceman would then have to use his powers to keep the villain at bay while also helping Metalman regain control of himself. The dynamic between the two heroes would be tested as they face this new and dangerous enemy.
One possible storyline could be that Metalman and Iceman are forced to team up during a mission. They start off not getting along due to their different powers and personalities. Metalman is more rigid and structured, while Iceman is laid - back. But as they face various challenges together, they begin to understand and respect each other, and eventually form a strong friendship.
One great Iceman fanfiction could be about his struggle to control his powers in a more in - depth way. Maybe it shows his training sessions with other X - Men, like Rogue, who also has power - related issues. Another could focus on his relationships, perhaps a romantic relationship with a new mutant in the X - Mansion.
In a different take, a new character could be introduced as the main antagonist. Maybe a criminal mastermind who teams up with the Joker specifically to target Robin. This new character could have a personal vendetta against Robin or see him as a key to achieving some greater evil plan, and the Joker is just a means to an end in this scenario.
It could be Kaguya Otsutsuki. Since she has powerful abilities and a thirst for power, she would see Kefla's power as a threat and Naruto's influence as something to be eliminated. She might try to manipulate events to turn the ninja world against Kefla and Naruto.
The Kingpin could be a great antagonist. He could be trying to take over more of the city and sees Spiderman and Tigra as threats to his plans. He uses his criminal network and his physical strength to try and stop them.
It could be someone who wants to merge their two worlds in a chaotic way. Maybe an evil scientist from Leela's world who has discovered the power of chakra and wants to use it for his own dark purposes. He could try to capture Naruto to study his chakra and use it to take over both universes.
The main antagonist could be an evil merman who wants to take over Ariel's kingdom and is also jealous of her friendship with Naruto. He might use his dark water - based magic to try and stop them.
It could be an old enemy of the gods who has a vendetta against both Camp Half - Blood and Camp Jupiter. This antagonist might see Percy and Reyna as threats because of their combined power and influence among the demigods. For example, a powerful Titan who escaped from Tartarus and wants to use the chaos between the two camps to his advantage.
An ancient evil force that was sealed away by Hashirama long ago could be released in Naruto's time. This entity, having a grudge against Hashirama for imprisoning it, would see Naruto as an obstacle too since he's allied with Hashirama. It could have powers that are completely unknown to the modern ninja world.
It could be a larger, more powerful wasp or bee species that sees them as a threat. For example, a giant robotic wasp created by an evil scientist that wants to dominate all insect - like beings. This antagonist would have advanced weaponry and superior strength compared to Waspinator and Killer Bee.