The most I interesting thing I have seen printed are bionic hands & also body organs. These will transform medicine forever.

If I could build my own printer I'd arrange for the filament to be interchangeable so that new products or printing material to the market could be added with ease, wifi capable, stop/start at the touch of a finger, expandable so you can print larger items if required, sd card accessible, works as a scanner as well as a printer, images of your own can be added & printed once converted to a suitable format.