Id buy a 3d printer
But the modelling software is a bit too fiddly for my tastes.
Id like something that is intuitive and simple to pick up.
Im sure some people here remember QOOLE for making Quake 2 maps...thats probably the easiest map editor ive ever come across.
You could build very complex stuff with that in a 3D space. It was very precise as well.
Something in that spirit would be epic.
Software to "test" designs would useful as well.
For want of a better example. Take the Raspberry Pi.
If I wanted to make a case for it, the ability to drop in a virtual model of that device to "virtually" mount it would be extremely useful.
I think we need a world where product manufacturers actively encourage the production of replacement components using 3d printers. I can see a market where you get the model files for replacement parts as part of your extended warranty as well as the support to assist in producing the parts.
Problem is, the likes of HP et al make a lot of money selling "maintenance kits".