Physarum sims are a really cool way to create these crazy organic looking shapes. I started with 2D sims, then tried with a sort of 2.5D rendering of those 2D sims. More recently, I've messed with volume pathtracing on the pheremone buffer of a 3D sim. I learned about this type of sim from this post by Sage Jenson. You can find out more in the articles below: