Well where do you start....
I guess the Unison for one and the power supply in a 19" rack mount,
individual CV\Gate outs for each voice and after touch VCF, VCA. And
the LFO mod for bank II, the heat-bar for the voice cards too would
be nice. Split the CS into two wood cabs (keyboard and panel in
individual cabinets). And if I had midi it would be only to sync the
LFO's Glissendo/portamento to midi clock. There is no reason to play
a CS80 from another keyboard because that just misses the whole point
of haveing one.