My medical mask fetish predominates, I think. In my fantasy world, the girl must be wearing an N95 mask or cartridge respirator or similar, and should be forced to do so, either by her doctor or by some self-diagnosed medical need.
Having said that, the other main fetish of mine that I have had my whole sexual life is girls in footed pajamas. I've told that story before. I was at a babysitter's house, and she instructed her two daughters to get into their pajamas before we all watched TV. One of them came out with a one-piece pink blanket sleeper with feet (and white toecaps on the feet, as was common in the 80's). My heart just slammed into overdrive. I didn't realize it was sexual back then (I was maybe 8, and she was 9). But now I definitely do. And so one favorite fantasy is a girl in her hospital bed at a hospital where you are allowed your own pajamas, and she is wearing footy pajamas and a protective mask on her mouth and nose (maybe an N95 or a Looney Tunes mask, or even like I said, a half-mask cartridge respirator).
My fetish life started with an attraction to girls in either oxygen masks or anesthesia masks (back then it was the black beauty), and that can still be a turnon at times.
Another big fetish for me, that can be very combined with masks, is the school outfit. But in this case, I don't like the skirt to be super short. I like it to be knee length, and I like the girl to be wearing opaque (preferable cable knit) white or red tights and well-polished black maryjanes. And maybe she has, say, a peanut allergy, so she needs to wear a mask at school.