I completed my Rocket fuzz a couple of weeks ago, and after playing around with it for awhile, it occurred to me, it'd be pretty fricken awesome to add a blue box-like 2 octave down effect to it. Would this be a possible as an addition, or would it be too hard to implement into an already completed pedal?
The blue box is a pretty complex circuit. It wouldn't be easy to add it to an existing pedal and I don't know of any simpler 2 octave down pedals. An octave up fuzz add-on would be easier. A green ringer type circuit or something like the simple octave up (http://www.generalguitargadgets.com/diagrams/simple_oct_up.jpg) would probably work well. You might even be able to get away with taking out the op amp stage.
This is one of the reasons why I like to remind folks about parallel processing, and the advantages of having splitters and mixers hanging around. They permit you to combine effects in exactly the way you describe.