1st one I plan to use as simple VCF with basic cutoff/resonance knobs and fit it inside a Casio CZ-101 I have to tinker with.
Another module I thought of making would include an ADSR, an LFO and perhaps use a PIC or Arduino to control them all via MIDI CC's.
I have several SSM 2056 ADSR chips from a Korg Poly 61 that may be easy to get quick responses from
DAC's are dirt cheap anymore.. I got two of these recently
PCF8591 AD/DA Converter Module Analog To Digital Conversion Arduino+Cable UL
very simply to control with Arduino via I2C