Rarity's Dischordia
I'm calling this concept "Dischordia".

The concept is pretty simple, in this the meaning of the Cutie Mark is flipped. It is no longer a destiny or a talent, but rather a dark desire that they are sort of a slave too. (This concept is for human ponies only, because I don't like dark ideas with actual ponies.) The ball joints are a way to show that the desire holds a power of them, sort of like they are just puppets for their desire.

Rarity's Cutie Mark is three diamons, therefore it signals her hidden desire for fame, talent, and semi-precious stones.

This is my design for Dischordia Rarity, and hopefully a future cosplay!