OK...attenuator/inverter/mixers are backbone stuff. There is a Shades in your build, but this only provides three channels of CV/mod manipulation. Better choices would be Happy Nerding's 3X MIA, Antumbra's ATN8, Frap's 321, Low-Gain's CVP1, Tiptop's MISO, etc. The key here is functional density...you have to cram as much as possible into as tight as space as you can where utilities and other "no-tweak" modules are concerned. This then frees up more space for the modules that require more "playing surface are", like VCFs, etc.
Mod sources...the Quadrax is quite good as far as complex function generator/envelope generators go. But while Tides is a great LFO-type source, you're better here if you find a smaller form-factor 3rd party version of it. Again, you need to cram function into tight space here. Better still: Tesseract has a dual Tides that fits into only four more hp than Mutable's original. Keep an eye out for things like this. And don't count Maths out; there are REASONS why that module sees so much use! Tony came up with a true classic there.
Keep the Sloths, also. If you're doing anything ambient, generative, etc...self-regulating systems, basically...having low-speed randomness is a must. However, it could be augmented with comparators, logic, and so on...things that can pick off specific voltage states, combine these with clock pulses, and develop entirely new behaviors as the system runs. I strongly suggest looking into both categories I've mentioned here, as they open up new control possibilities for your modulation sources to work with.
Now, VCAs...the red-headed stepchilds of Eurorack, seems like...there are technically two: linear and DC-coupled, and exponential and AC-coupled. The latter is purely for audio, because these manipulate signals in ways that our hearing recognizes as "changes in apparent loudness", plus they BLOCK DC...which is really effin' critical if you have an amp that's DC-coupled, because if you send enough DC to that amp by accident (like passing a DC offset to your output...oops!), you won't have speakers if you keep it on long enough at a high enough level! The ARP 2600 was infamous for this. But the linear VCAs are key to manipulating CV and especially modulation signals. Since they can pass DC, they can be used for audio (preferably if you can send them exponential envelopes) OR subaudio signals down to DC. This means that if you want...oh, say, a rising amount of vibrato on a single VCO after a hard-attack audio envelope opens...well, you can do that. Just trigger the EG for the audio VCA and another one for the linear modulation VCA, set the first with a hard attack and a medium tail, and the second with a slow attack and medium tail, and there you are! One key press does it all.
I've repeatedly said that YOU CANNOT HAVE ENOUGH VCAS. I ain't kidding! They have ridiculous amounts of uses, particularly if you're talking about VCAs such as in the MI Veils or Intellijel's Quad VCA, because those not only pass DC, you can tailor their response curves. And that opens up a whole other pile of interesting possibilities. But here again, cram these as tightly as you can! And make sure you also get mixer capabilities, because mixing/adding CV and mod signals...yup, more fun potential. MY big choice for a dedicated linear VCA bank, btw, is the Erogenous Tones VC8...8 VCAs in one, with two breakable/linkable mixer paths. But for variables, Intellijel seems to have that down. Also...when you get a chance, try using an audio signal as a CV for a linear VCA while feeding another audio signal through it. Like I said, ridiculous amounts of uses...
Now, downright missing stuff...first up, look into waveshapers. These allow you to do loads of timbral manipulation before you even get your oscillator signals to the VCF, and make for a cheap way to get a more complex sound. In a similar vein, suboctave dividers are a super easy and effective way to create doubling...particularly into sub-wrecking subbass frequencies. This one point was/is why the Roland SH-101 punches as hard as it does with just a single VCO. And another key thing: ring modulators. Hell, I have "raw" ringmods permanently patched into my routing bays here and feed 'em with sine generators so that I can "whang up" any signal I damn well please! They're a big key to getting strange, alien, metallic sounds, but you can also use them as tremolo circuits (with an LFO as "modulator") to vary amplitude...just like a VCA, kindasorta.
Now, stuff that's just downright wrong...for one example, let's start with that Roland/Malekko mixer there. Since this is a smaller build, you should consider using mixer level VCAs to control your final signal amplitudes...but the Roland 531 only has CV over panning. You could use those as level VCAs (kind of) but then you'd have to run the mixer in mono, which defeats the purpose of having a stereo performance mixer. Plus, if you're properly submixing within your patches, you probably won't need six input channels at your output stage in this small a build. But if you step up four more hp, then you find the Toppobrillo Stereomix2...which gives you CV over level, panning, AUX send, plus you get channel muting and an FX send/return path. And this at only $80 more than the Roland. True, you lose two channels and your 1/4" outputs, but what you gain here is immense. Plus, if you want a balanced output AND a second stereo AUX return, you can pair this with a Happy Nerding OUT...so you get transformer balancing, metering, your headphone amp, and...yep...that second parallel stereo input.
Which brings up a point: sometimes the solution to a single issue is NOT a single module. In fact, much of the strength to be found in modular synthesis in general is in module complementarity; sure, you could add a second VCO for timbral variation...but you could ALSO add a waveshaper along with it, and then you've got tons of that timbral variation with just 12 more hp (or less...) used. As you remove and replace the above, try and focus on what combinations of modules do. Then how do those combinations work together as subsystems? And so on...as this is the key difference between a pile of electronic crap in a box and a real INSTRUMENT that you can work and live with for potentially decades. This is why I'm saying "go back, strip out the junk, redo"...because no one wants to drop several grand on something that you'll have to keep dropping several grand on. Instead, proceed slowly, carefully, and after studying the hows and whys of this stuff, proceed precisely so that you can hit a result that needs nothing but which provides everything. And MG's no video game...you don't have to get this right inside a time limit or hit a specific score. You have the room and the tools here...but TAKE THE TIME, because that's the one thing you bring to this process. And there's no substitute for properly using that resource when you're dealing with building an instrument that should be on-hand for decades.
