Another possibility is that it could be a jealous character within the Kim Possible world itself. Maybe someone who is envious of the attention the cat girl is getting, or feels that her presence is disrupting the status quo. This character could then start causing trouble and become the main antagonist, adding a more personal and internal conflict to the fanfiction.
It could be an evil scientist who wants to capture the cat girl for her unique abilities. Maybe he wants to use her enhanced senses or agility for his own malicious purposes.
Perhaps a new villain from a different dimension who sees the cat girl as a threat to his plans of taking over the Kim Possible world. Since she has cat - like abilities, she might be able to thwart his initial attacks, so he makes it his mission to get rid of her. And this would set up a great conflict for the story.
A new character could be introduced as the main antagonist. Maybe a powerful alien who sees Kim's new powers as a threat to his plan of taking over the earth. This alien could have his own set of powers that are quite different from Kim's, like the ability to control minds. Kim would have to not only learn to use her new powers but also find a way to defeat an enemy with such a dangerous ability.
Shego could also be the main antagonist. Maybe she discovers a way to time travel on her own and decides to go back in time to eliminate Kim before she becomes a threat. Kim would have to follow her through time to prevent this and save her younger self.
A possible main antagonist could be a rogue scientist from Kim's world who has somehow gained knowledge of chakra and wants to use it to create an army of super - soldiers. He kidnaps some of Naruto's friends to study their chakra and it's up to Naruto and Kim to stop him.
It could be a group of rogue ninjas who are after the cat girl for some special power she might possess. They see Naruto as an obstacle in their way to get to her.
One interesting aspect could be the unique take on the Kim Possible universe by introducing a cat girl. It might bring new character dynamics and storylines. For example, how the cat - like abilities of the character could be used in the action - packed scenarios that are typical in Kim Possible. Also, the design of the cat girl's appearance and how it fits into the existing aesthetic of the show.
A powerful sorcerer who wants to enslave all the insects for his own dark magic could also be the main villain. Mosqito Girl would have to use her wits and powers to stop him.
It could be an envious rival. Maybe there's another girl who is jealous of Pyro's unique abilities or charm. She constantly tries to undermine Pyro and create obstacles in her path.
A jealous witch could be the main antagonist. She sees the Sunlight Girl's popularity and power, and out of envy, she tries to cast dark spells on the girl to dim her light.
Perhaps a character who is allergic to balloons and wants to ban all balloon - related activities in the town. This would put the balloon girl in direct conflict with them as balloons are her passion.
Perhaps a stray dog. The dog is always looking for something to chase or bother. When it sees the mommy cat taking care of the baby robin, it decides to make trouble. It tries to scare the cat away from the robin so that it can have fun chasing the small, helpless bird.