The closest to "passive income" you'll likely get is stock sites like Shutterstock & iStockPhoto.  If you want more info in that direction, let me know.  I used to write a popular microstock blog but gave it up because it's not *quite* passive enough for me. :)

Use your powers of design for good! Your design education is like a secret sauce the business world seeks, yet most designer limit their ambition to the Red Ocean of highly competitive markets. It will be wasted on existing businesses.

Take Geetika's story as an example of how the design mind can be put to use by being truely creative: http://thecreatorsproject.vice.com/blog/vacation-... There are a plethora of examples. AirBnB are designers too.

The world is craving new ideas, creativity. Most business decision makers are terrified of what they don't understand, like creativity. So designers need to be the change, we seek. Starting a business is defiantly not passive, but with the right strategy it could give you freedom from the project-to-project trap.