The author of that book basically dreamed up skills that could possibly be done. You can do the same thing. Think of some skills. Here's an example: Toe hecht to handstand on the high bar. Not possible, you say? Doesn't matter. This is brainstorming....hmmm What about toe hecht off the high bar to handstand on the low bar! Now that could be doable! If you want to see if any of the skills you dream up actually exist, check the code. Or you could do the opposite. Check the code for skills that are already in there, then have your gymnast do a similar skill. Find out the rules for getting skills named after gymnasts in the JO code. I think the FIG requires skills be a certain value to be named after a gymnast. I don't know if the same is true for JO. Also remember that the JO code and the FIG code are different. You can look for skills in the FIG code that appear to be missing from the JO code and get a skill that already exists named after one of your gymnasts.
The book is called "Flip outside the box" by La" by Lurie Gallus