Hey there!! You're welcome authorcchi ^^!! I like your story, especially when you added some benders ♪(๑ᴖ◡ᴖ๑)♪ and about what should happen.. Honestly I'm bad at these things but, I will give it a try.
I think that before the black dragon joins them, there should be a new enemy that prevents the black dragon from coming with them, and that enemy turns the table around, like instead of the black dragon thinking of them as allies, she will think of them as enemies.
Or, while they're at the village of the black dragon, the purple dragon will remember something, that will let her lose control of her powers. Then, unconsciously trap herself or her comrades in her own illusions, and while that is happening, she will accidentally do something that will make the black dragon lose her trust or something like that (๑˃̵ᴗ˂̵). That's the only thing I thought of authorcchi _| ̄|○