Improve the accuracy
It is possible to improve the accuracy of the printer by using lead-screws which are more accurate than timing-belts, but at the expense of speed. It would also be a good idea to replace the stepper motors with servo motors to avoid vibrations. We believe that a vibration-free printer could draw tracks of 0.1 mm or even less.
Support positive photoresists boards and sprays
There are positive photoresist boards and sprays in the market that could be used for the printer. This requires spend some time in Eagle to find how to print in HPGL the gaps between the tracks instead of the tracks and pads themselves. We believe that this could be done developing a program in the Eagles's User Language (ULP) in order to mirror the image and generate the new file.
Currently the printer only supports HPGL file format. An interesting improvement would be support Gerber format. The main problem here is the Arduino ONE's shortage of memory. To support Gerber probably you have to migrate the hardware and firmware to Arduino MEGA.
You can redesign the printer bigger or smaller, it's all a matter of lengthening or shortening the linear guides, belts, and reconfigure the firmware. If we would build it again, it will be smaller and more precise.