Agreed. Take out the working cartridge of a normal pen, along with the tip, and create a hand-grip, as well as a hatch at the back to replace the cartridge. So many ideas... you could also use multiple cartridges with different colours, or all of the same colour (in case one runs out) and have a switch system to swap them out. The problem is finding the right pieces, and having it an appropriate size for your hand. I hope I have contributed in some way. Feel free to use my ideas if you find them useful.
May the Lord guide our creative minds to prosperity.