Another option is a new character created specifically for the fanfiction. This character could be jealous of the bond between Naruto and Aela and wants to break it apart to gain power. They might manipulate events and other characters to achieve their goal.
It could be a powerful ninja from Naruto's world who wants to use Aela's powers for his own evil plans. Maybe he thinks her abilities can help him conquer all the ninja villages.
Perhaps an entity from Aela's world that has followed her to Naruto's world and starts causing chaos. This entity could be something like a dark spirit that feeds on destruction.