Even though I already replied to this thread, I feel the need to say, "Shoulders" again.
Because gawd, I hate shoulders. Every silky robe and armorkini design ever... handily ruined.
I would love it if you could one out of two shoulders invisible. Would create a lot of fun set customization.
Hands. There are some pretty cool bracers on the game and I rarely get to see them =P
Torso would be nice too, but I think it's kinda of cool to challenge yourself into doing a "savage" transmog and so many people would abuse of this it would lose its fun factor in 2 weeks.
I always thought Transmog ruined my feeling of working hard for something and displaying it, no matter how ugly. Reason why I don't use it.
However, I think it is still a cool feature for that occasional piece I can't stand, also just for RP reasons. I'd love a invisible transmog option for ALL pieces, also tattoos would be sweet.
I have yet to see a belt that didn't clash horribly with my transmog. It's really annoying that there are next to no minimal belts anymore and those that are, aren't in the colors I need (purple or orange-red mostly, but sometimes fel-green. Yeah, I'm a warlock.)