Fetish; pantyhose, nylons & stockings, sheer bodystockings.
I'm pulling Sicario into the costumes group.
Also, when I think of costume porn, I also think of period/vintage shoots and scenes. Do a Downton Abbey or Boardwalk Empire themed porn movie. I know it was a commercial flop, but I enjoyed Bob Guccione's
Caligula. Just make something along those lines, minus the crazy production budget and bigger name, mainstream actors on the side. More stuff like that and less stuff that's just a recycle of standard porn scenes that have been done since the 60's: maids, pizza delivery, driving around picking up people, pirates, aliens, etc.
And how about a Twilight Zone/Walter Mitty type porn movie??? Ya know, one where someone starts daydreaming in a mundane office or work environment
(we could all relate to that, right?!) and the scene is about what they're fantasizing about... and then they get snapped back to reality. You could take that anywhere and make it work for men or women, depending on what audience you're trying to reach.