If you want to head in a Buchla-type direction, toss the filters and go with LPGs to get the simultaneous dynamic/timbre control. Also, add some sort complex oscillator, and that also heads you that way by getting the FM-type thing going.
For Serge without Serge, having the MATHS is good, but adding a couple of VC Slopes of some sort just by themselves to serve their own multiple capabilities would be a good idea. Also, I would consider ARC's Serge-type mixers as being more sonically right plus a better use of 'real estate'.
In either case, hunting down some sort of colored noise source with variability would also be key. Also consider a sort of 'Source of Uncertainty'-type module as a way of adding some weird psuedorandomnesses.
By default, even if you went in a heavily 'Buchla' direction, also, you wouldn't have 'exactly that' in Euro since you have only one signal patching format, not Buchla's separate control and audio patching, so going back and forth between those paradigms would be very easy, where on Buchla (200...not necessarily the 100 Series) it's not that nicely-done.
More VCAs, also: linear and DC-coupled for processing CVs. This along with the Wogglebug would be entertaining...
Lastly, sequencer. A must-do for both modalities, but you have a lot more potential methods in Euro to fit your usage idea. Turing Machine's not bad here, but also consider something a bit more 'determinate' as well.
Good start; have a look at the Elby stuff for more things Serge-like, as well.