One possible antagonist could be Orochimaru. He might be interested in Kim's unique skills and gadgets for his own evil experiments. He could set a trap for her and Naruto would have to save her from Orochimaru's clutches. Another could be Drakken. He might team up with some ninja villains, like the Akatsuki members, in order to achieve his own goals. Naruto and Kim would then have to face this combined force.
Itachi Uchiha could be an antagonist in a different scenario. Maybe he is under a mind - control spell and is sent to capture Kim for some unknown reason. Naruto, who respects Itachi but also knows he has to protect Kim, engages in a battle with Itachi. Naruto tries to break the spell on Itachi while also keeping Kim safe. This creates a complex situation where Naruto has to use his wits as well as his strength to save Kim from Itachi's attacks.
A new villain could be created specifically for this fanfiction. This villain could have the power to manipulate both technology and chakra. He targets Kim because of her reputation as a hero and Naruto because of his strong chakra. Naruto has to save Kim from this villain's initial attack and then they figure out how to defeat the villain together, using Kim's knowledge of technology and Naruto's ninja skills.
In this fanfiction, it's possible that an ancient and powerful beast is the antagonist. The beast might have captured Pyraa to lure Naruto into a trap. Naruto will need to use all of his skills, including his Sage Mode if necessary, to defeat this beast and save Pyraa.
Maybe Kim Possible is trying to stop a global threat that has ties to the ninja world. Naruto discovers this threat too as it endangers his village in some way. He rushes to help Kim, who is already in the thick of the battle. Naruto's ability to summon toads gives them an edge in the fight. Kim, in turn, shows Naruto some of her cool gadgets and they work together using both ninja techniques and high - tech devices to save the day.
One possible antagonist could be the Akatsuki members. They are always looking for strong opponents or valuable information. Maybe they target Iruka because they think he has some knowledge about Naruto that they can use, and Kakashi has to stop them.
Danzo could be a main antagonist. He was always paranoid about the Uchiha clan's power and was involved in many of the events that led to their downfall. In a fanfiction where Naruto is trying to save the clan, Danzo would likely oppose Naruto's efforts as he sees the Uchiha as a threat to his own power and the village's stability in his own twisted way.
It could be some dark magic users from Lucy's world who see her as a threat. They might be trying to stop her from using her magic for good.
One possible antagonist could be Orochimaru. He has always been interested in Naruto for his Nine - Tails power, so it would make sense for him to be a threat that Kakashi has to protect Naruto from.
One possibility for the antagonists could be a rogue group of ninjas who are after the power of the Sound Four. They might see the Sound Four as a means to gain more influence in the ninja world.
One possible antagonist could be a rogue ninja from another village. They might target Tenten for her unique weapon - handling skills, thinking they can use her to create powerful weapons for themselves. Naruto would then have to face this ninja to save Tenten.
It could be some rogue ninja clans that are after Karin for her special abilities. They might see her as a tool to gain power or information. For instance, a clan that specializes in blood - related jutsu might want to use Karin's unique blood - sensing ability for their own evil purposes.
One potential antagonist could be a power - hungry shinobi from another village. They might view Hayate as a threat to their plans, maybe because Hayate is involved in some sort of peace - keeping mission between villages. So, they try to eliminate him. And Naruto, being the defender of his comrades, will not stand by and let that happen. There could also be a group of missing - ninjas who band together to take on Hayate, thinking he's an easy prey, but they underestimate Naruto's loyalty and strength.