Just a short suggestion:
The idea of choosing which parameters to randomize is very good, and could be supplemented with another function: For the operator frequencies, one might have the option to only produce random values which give rise to "nice" frequency ratios, in order to produce more "musical" sounds. (Easy to implement for one carrier-modulator pair, perhaps trickier when it comes to a higher number of operators, I have not really given this much thought.)
