The one's I've tried haven't been the most interesting... although I've tried relatively simple ones like (z^5-1)/(z^4) using an a value of 1/(n-1) (in other words, leaving out part of the derivative to get it to make an even remotely interesting shape).
The problem being they are weirdly shaped, and not all that fun to navigate. Perhaps some of the equations will be awesome.. but I've yet to find one- I've found that non-newtonian, in other words not strict z_n+1= z_n - a* p(z) / p'(z), formula variations make some interesting things... but still.. bleh. More work to be done. Here are a couple boring ones for you to gander at (keep in mind to see any decent details you need to slice the fractal, as it has whole sections of boringness that block details):