Thread: Boards of Canada style rack.
You could do an FX rack and process one of your hardware/software synths through it. There's lots of really capable effects in eurorack and you can get creative with custom feedback loops and modulation without having to hook up a million different pedals. Like everyone else has said, eurorack is an expensive way to create a polysynth that's probably disadvantaged compared to regular polysynths, but sticking to FX, samples or monophony might scratch your itch for sound design.