How about an amplifier with its gain programmed by the voltage on a dedicated pin?
I don't mean a PGA with a few discrete settings (eg. x1, x5 or x10, take it or leave it), but continuously variable gain from 0 up to some defined maximum, with either a linear or log response in between.
Old school: LM13700 (or even older relatives!), current controlled. Gain is also proportional to absolute temperature.
Newer: VGAs are common for video applications (oddly enough, for the name, I guess?). Example:
http://www.ti.com/lit/ds/symlink/lmh6505.pdfGain still varies with temp, possibly by the same figure.
There's also internal-ADC types like the AD603 that do it in steps, but fine enough you may not care (<1dB increment isn't much to fret over).
Along similar lines, a nice, simple filter IC with its cut-off frequency adjusted in the same way would be nice. No need for multiple, ganged external pots that all vary together.
Peruse some old school analog synth schematics. You'll find piles of OTAs like the LM13700.
Also, JFETs used as variable resistors (usually with op-amps), or CdS photocells paired with LEDs (aka vactrols). (Or before LEDs, neons or incandescents were used too.) JFETs are generally hard to use at high linearity and dynamic range, and have to be sorted for matched characteristics. CdS have weird time constants and poor repeatability. Such circuits were huge in compensation components (thermistors and diodes and etc.) and had many trimpots to calibrate.
The better alternative for filters is a switched-capacitor type, many of which are available as ICs (I think LT and Maxim are the most important producers of these). Cutoff is proportional to clock frequency, which can easily be varied continuously with a VCO.
Tim