Ralph points out a shortcoming of nearly all current 3D printers, which is especially acute in FDM machines: the flat planes they print in. This means that while a part may be strong in the X/Y directions, its strength in the Z direction is limited by the adhesion between a hot layer on top and the cool layer below, which is typically rather weak. The way around this is to devise a machine that can truly move in 3 dimensions, applying material across vertical layers. So if you picture a bowl being printed, for example, the outside might be printed conventionally, but then a second layer could be printed over that which rasters cross-wise over all those layers from one side to another, swooping up the sides of the bowl and across the bottom. That would give it a lot more strength. But it would require a longer nozzle than current printers have, and the ability to swivel it. A robot arm could certainly do it, but a more conventional 5-axis machine (or a delta) could do it as well. Keeping the plastic heated in that longer nozzle would be the biggest technical issue that I can forsee.