I'd like to see Rarity as having limited vampiric powers but not overpowered. Also I like the ability of her to use genjutsu. But her greatest skill would be in her cunning trickery.
This song would be her battle theme:
"_Nightwish-Planet Hell _":https://m.youtube.com/watch?v=JtxyeVdSEms&list=PLaKGqsgrt0cI3FyY-h8U54DXJf2wQbtkO&index=5&t=0s. This is the song that plays when she has to go all out to defeat an opponent of epic power.