Recently I tried to recreate the sound of an old C64 track using Live. I failed, because of simple functionality which could easily added and would be a great improvement of Live's synthesizer. I split this topic into two forum posts, one for each feature request. I programmed own music composing software for C64 (some time ago), and was a little bit surprised why this things never got into modern synths.
A: Control of Operator Rectangle Shape
C64 musicians had only sine, triangle, saw and rectangle waveform shapes. But the rectangle shape was very flexible, because there was a register to control the "shape" of the rectangle waveform. More specifically the ratio between the "up" and the "down" part of the rectangle shape.
See this illustration:
Controlling the shape of the rectangle waveform using an LFO, is creating very unique effects. It shouldn't be hard to add this to the "Operator" synthesizer, without breaking backward compatibility.
I created a very simple C64 Arp synthesizer, using Max for Live, which demonstrates the effect:
https://www.dropbox.com/s/2702orl71rjs3uc/dzc64ap.amxd
You can hear the synthesizer in this sample track on soundcloud:
http://snd.sc/14e5nKx