Yeah I thought that too, bring in the external VCO on the mixer, roll up the cutoff on the units internal filter so the signal passes through, then maybe route adsr 2 out to the external filters control, and finally, the whole sum audio output goes through the external filter and then on to the mixer.
See this way I could build up some really cool eurorack oscillators, eg polivoks, zero thru, Wavetable, etc, some cool filters, a couple more lfo's, without needing to buy dull things like envelopes and mixers and blah blah blah. And I could go at my own pace, start with an already fully equipped synth and build from there