Things that would be nice, are not "complete" systems, and are hard to find in FIRST quantities/sizes/weights/prices:
Aluminum gears/pocketed steel gears
Slip rings
Pneumatic brakes/clutches/ratchets
118-style floating idlers (hard rubber or metal) for #25 and #35 chain
Harmonic drive gear boxes (
http://en.wikipedia.org/wiki/Harmonic_drive - good luck with this one!)
Differentials
Ball-lock shifters
CVTs (dual cone type)
IVTs (planetary gearbox with driven sun and ring gears)
Ball casters that are resistant to debris
Suspension components