How about a VU meter/output module (ala Roland System 700 and EMS),
or a frequency counter (once again like EMS Synthi 100), or even
passive attenuating mults (like the old Digisound Processor module).

Personally, and without trying to detract in any way from those that
may disagree with me, I think these modules would be far more useful
to a synthesist than a Nintendo controller.

But that's just my two cents, which is worth exactly... umm, two
cents.