Depends on how you define "fetish": "a form of sexual desire in which gratification is linked to an abnormal degree to a particular object, item of clothing, part of the body, etc."
@DeadlyDuo 's comment, "He really went to town on Drusilla who was "chaste and pure". So maybe corrupting innocence?" is, IMO, fetish in this sense.
We learn in A11 that his father feared he'd corrupt his younger sister, Kathy (I've phrased that badly - I don't mean incest - more make her evil). There a thread through from Kathy to Drusilla and then to Buffy (Innocence). Kathy also put him under a mandate to remember her - so Kathy and Buffy are linked through IWRY. I'd say "Innocence" was the fetish. Corruption is a conscious act by Angelus and an unconscious'repressed fear of Angel.
The second meaning of fetish is "an inanimate object worshipped for its supposed magical powers or because it is considered to be inhabited by a spirit". Personally, I think Angel fetishizes his soul... and his father. Doesn't he masquerade as a priest (a father) to corrupt Drusilla? I'm guessing much of this can be linked through Freud's Totem and Taboo (I know Whedon's supposedly a fan of Joseph Campbell and I think Campbell's influenced by Freud and Jung).
Angel has said he has a thing for convents (kinda goes with your nuns).
Can I just point out that Nuns are called "Sister"?