@Coalman Thanks for the feedback. I think it's "cpu intensive" in a different way than audio dsp which is sort of a constant load, here is more of a peak as far as I can tell and might be because the lines grow exponentially with each iteration and there are a lot of elements to parse, so perhaps that's why the cpu needs more time.
This stuff is all new to me so I'm just trying to figure out how to implement some of those algorithms I see around, and see if there's a use for them. Here I found some good explanations of the various types of L-systems: http://www.modularbrains.net/support/SteliosManousakis-Musical_L-systems.pdf
The non-propagative systems seem very interesting too.
With regards to your suggestion about "setable loopbacks", do you mean to have the ability to pick two points in the string and use the portion contained within as the new seed for the next iteration? Perhaps this could be done by doing iterations element-by-element rather than string-by-string.
One thing to point out is that, as far as I can tell, if you put a string within brackets for example, that string will effectively become a branch with brackets as delimitators (and will have sub-branches defined by sub-brackets). The fractal_tree example in my patch is an example of this. But maybe what you are suggesting is to export the different branches, or somehow isolate them. Am I correct? It's interesting, I'll think about that.