Courtesy of Mark Lentczer, have a gander at Context Free, an implementation of Chris Coyne’s design grammars. This puts me in mind of Alvy Ray Smith’a graftals or L-systems which I first read about 20 years ago.
After a few minutes of playing, I came up with this:
A few minutes more hacking resulted in the following branching structure (click on it to waste my bandwidth and download the 2000×2000 image):
