I came up with a device today that I want, but I don’t think exists.
You would feed in some kind of form - like an employment form or check. The device would scan the form and display the scan on your monitor. You could then point to different spots on the monitor display and enter text. When you hit save, the device would feed the original document through a printer that would print on the original what you had entered on the screen.
All of the technology exists right now, except perhaps for the feed from the scanner to the printer.
What would you like to see invented?