Thanks for the replies guys, very interesting.
It IS early days but I did wonder if we were early enough to be at the top of the curve of these types of domains - but google search volumes indicate this may not be the case.
The idea that it is just for designers to produce mock ups/prototypes is far from the truth. There are some industries that 3d printing has already made a big difference - in cost, speed, simplicity. I'm not going to say it's mainstream, it's not, those industries are niche no question but it IS making a difference.
The directory idea is just about the only thing I can think of developing, only issue is right now my job leaves me no time to really push any of my own projects - unless I wish to be single!