On the practical side, I'm probably going to need it to be much longer than average, so whatever I get either has to be extra-long, or something where I can attach more fabric to the bottom, in an aesthetically pleasing way. From my research, it seems that lightweight and/or canvas fabrics are the best for using without a liner, so I'm abandoning my idea of silvery-grey velvet drapes -- too bad, would have been dramatic. :-)
Some of the options so far -- and again, this is mostly brainstorming, as I may also end up sewing something out of cool fabric -- Spoonflower has some fabulous options for both steampunk and medieval:
Noble Pheasant (it's hard to see in the photo, but if you click through, you can zoom in on the subtle details of flowers and birds), Anthropologie, $150:
Octopus, Thomas Paul, $96:
Indian-style, just an image, no idea where the actual curtain is from, but I think I could fairly easily sew something similar out of sari fabric in a more neutral color, and fairly inexpensively too:
Another Indian one from Saffron Marigold, this one block-print rather than sari fabric, $60:
Crewelwork Shower Curtain (this is my favorite of the batch -- it's hand-made, inspiration for something I could in theory do myself, if I learned how to do crewelwork first and somehow magically found aeons of free time between now and next March):
Any other suggestions gratefully welcomed. We do have four months for this decision, so it's not urgent, but somehow, it's where my brain is this morning. Shower curtains!