I think the reason a good editor is hard to create is that each operator plays a significant role in the sound and a different role depending on the algorithm.  In the subtractive sense, an operator takes the part of an oscillator, a filter, an output amplifier, a wave shaper or an lfo and could also be only one of many.  To limit an editor to one operator at a time is like limiting an editor to only one of those parts at a time.
  I'm not sure you could improve the PreenFM editor much at all without adding a significant amount of knobs.  It's quite easy to use with a printout of the algorithms in hand.  Though, eight knobs might be a bit better than four.  

  For a 'transition layer' you'd have to determine what operator played what part and was important based on the output.  Sounds like a significant amount of work.  
