Monster transformations (vampirism, lycanthropy, Frankenstein ponies ect.) because that sort of stuff has always fascinated me. I'm also rather fond of ponies getting corrupted or mad with power because it brings out the delicious ham in them. This is why I want to see Nightmare Rarity on the show, to watch her absolutely devour the scenery.
On the more perverse side futa is always fun, and I'm also partial to centaurs and anthro for naughtiness purposes
@gasmaskangel Yeah, Nightmare Rarity was a dream come true as far as fetish fuel goes for me. Though i wonder, if in the rare ultra unlikely case they ever do feature her in the animated series, would her voice just be the same as nightmare moons? (given that Nightmare moon has the same voice actor as rarity. :P)