I decided to split the Project into three seperate Boosterpacks, called "synth", "knobs" and "midi" to provide higher granularity. You can find the schematics in my github, also I uploaded the board files. Since I am just begining to work with eagle, my board design is far from perfect, and this is to be considered WIP. If you want to help with the layout, please send me a pull request.

Seperate BoosterPack details:

1. Synth
   1. 12-bit DAC
   2. Twin-T Filter
   3. PWM Cutoff control
   4. Resonance control via digital potentiometer
2. Knobs
   1. 8 Potentiometers
   2. 4 Switches
   3. ToDo: Add multiplexer to save pins, maybe a tiny display
3. MIDI
   1. MIDI In/Out
   2. Gate/Trigger In/Out
   3. PWM CV Out
   4. Everything provided as 3.5 Audio Jacks
   5. ToDo: Add ADC for Audio or 1 V/Oct input
   If all three Boosters are finished, it should be possible to stack them to get a complete Synthesizer.

Comments and suggestions are very welcome.