I would get some mults and other utilities perhaps S&H and more envelope generators? Perhaps start with a few core modules and build from there? I am starting with just a basic Doepfer A100 system and adding to that over time. Dead simple with VCO, filter, envelope generator (EG), dual VCA. I did add more VCAs, mults and another VCO and LFO to my build plus a quad EG/sequencer and two mixers to have choices to route send/receive of sound sources.