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.
Well, perhaps she gets the power of invisibility. At first, she uses it for fun, like sneaking into places she couldn't before. But then she realizes that there's a villain who is also invisible and she has to use her new power to track him down. This could lead to a whole new set of adventures where she has to rely on more than just her physical combat abilities. She might have to use her wits in a different way to outsmart an enemy she can't see.
Maybe the main antagonist is an old enemy of Robin's who becomes jealous of his new powers. This enemy then devises a plan to steal Robin's powers or at least neutralize them so that he can regain his former status as the top threat. He could be a powerful martial artist or a criminal mastermind with a large network of henchmen.
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.
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.
It could be another character who is jealous of Lucy's new powers. Maybe a classmate or a rival in her community who wants those powers for themselves and will do anything to take them from Lucy.
A dark entity from another dimension could be the main antagonist. It senses Anna's new powers and wants to use them to open a portal between dimensions so that it can bring its army of monsters into Anna's world. So it constantly hunts Anna down, trying to either force her to use her powers for its own evil plan or simply kill her and take the powers for itself.
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.
Another possibility is that one of Leo's classmates becomes jealous of his powers. This classmate could start spreading rumors about Leo, trying to turn everyone against him. Eventually, the jealousy could turn into something more sinister, and the classmate could team up with some bad elements to try and take away Leo's powers.
Some fanfictions might give Naruto the power of elemental fusion. He could combine fire, wind, water, earth, and lightning in ways that are not possible in the original series. This would make his attacks extremely powerful and unpredictable. For instance, he could create a storm of lightning - infused water that not only electrocutes his enemies but also drowns them.
Kim's ability to quickly adapt to different situations and Naruto's immortality would make them a formidable team. If Naruto were to be in a dangerous situation where he might be killed, his immortality would keep him alive, and Kim could use her wits to get them out of the situation. Their powers would complement each other in various combat and problem - solving scenarios.