Well, one specific fetish that involves Rarity that has VERY little focus is the subject of her indulging in being a ‘mule’, the origin of which goes back to “A Dog and Pony Show”. I’ve only seen two images on this site that play on this fetish: one where’s being fucked doggy-style and spanked by Rover, and one where she’s alone, sporting mule ears and tail, with her cutie mark crossed out and replaced by a mule face. They’re both okay, but still, there’s one part of this fetish that I have yet to see.
I swear, I would cum in less than a minute if some artist would create this: a comic of Rarity being fucked by the Diamond Dogs, fully embracing her role as a mule and…either at the request of the Dog or of her own free will…toss back her head and fucking BRAY!!! To have the most uncouth of noises come out of her mouth, to completely lose herself in the roleplay, that would be a sight to see!!
An ever rarer Rarity-themed fetish I haven’t seen at all here, yet once somewhere else, is something I could only call a ‘rending’ fetish: to get off on seeing clothing (preferable clothing that she made herself) ripped apart, soiled, or otherwise destroyed.