One hurdle you may encounter is EPA, since repellent products with repellent claims might need to be registered with the agency, a process that usually requires submission of efficacy data.  So one question right away is:  what pest do you wish to repel?  Ticks?  Mosquitos?  Fleas? Small children with sharp sticks?  And then:  what are the acceptable tests that can confirm efficacy?  There are lots of claims made for various natural materials, but as you might imagine, some work, and some flat out don't. 

A good source of inspiration, at least for repellents that work on humans, is the USDA Lab in Gainesville, FL.  They have been working on repellents for years (we have them to thank for DEET), and would have a good grasp of what natural materials might work.