For instance if you have a full escape-time IFS for the branches of a bush but on reaching a given overall scale factor down any path through the IFS tree you switch the transforms to ones that produce a leaf, or (conditionally) a berry....
Ideally a completely generic fractal program would allow the user the option of fully defining the entire IFS tree i.e. number of child nodes for each parent node and the relevant transform to be applied (linear or non-linear) to a specified depth (e.g. termination of a path by specifying 0 as number of child nodes).
Some time ago I developed Structure Synth, which makes it possible to build 3D systems along the lines above: for each node you can have a number of child nodes and transformations (only affine transformations though). Nodes may refer to each other recursively (allowing for fractal structures), and nodes may have multiple definitions with different weights (allowing for random systems depending on the initial seed). These systems were specified as Context Free Grammars. The approach is very similar to stochastic Lindenmayer systems.
There is some more info here:
http://structuresynth.sourceforge.net/index.php and some example images here:
http://www.flickr.com/groups/structuresynth/Of course my approach did not use implicit surfaces, but instead explicit polygonal geometry (each node must terminate in one of the available graphics primitives: box, sphere, point , ...), so you quickly run out of polygons this way - and you don't get the cheap sphere ray tracing :-(
Being able to transform these kind of systems into a escape time / distance estimation system would be great, and I have thought about it several times. But I doubt it is possible. Systems like the KIFS are able to produce an exponential amount of objects given a number of iterations, because of the intrinsic symmetries (even though the symmetries are no longer visible, because of rotations or other transformations inside the main loop). I doubt generic IFS or L-Systems can be cast into a similar symmetric form.