Role-playing game often have complex hierarchies of skills that characters can acquire. Documentation of those skills are often in the form of a "skills matrix", which shows which skills are pre-requisites for others and generally which order they are acquired. I've often thought what a "skills matrix" for FAA pilot ratings would look like. I have posted a first stab at it.
Many of the ratings are not strictly sequential, but I think it's a reasonable representation. I've marked the rating that I'm working on and ratings that I might work on in the future.