I think about this a lot, I put a lot of effort into making my main system cohesive/“complete”. a few examples come to mind which I think can work for almost any case:
- functional grouping (similar modules close together, e.g. oscillators, filters, dedicated modulators)
- signal flow (arranging modules so there is a tendency to patch left-to-right)
- affinity grouping (modules which work well together close together)
- grouping by manufacturer
- grouping by color (panel, knobs, etc)
- as an alternative to grouping by color, the opposite: alternating colors to achieve visual balance
- grouping by west-coast/east-coast-yness
- symmetry (reflection, translation, rotation) in terms of function, color, etc.
- alternating wide and narrow modules
- arrangement by patch points, e.g. Cwejman modules tend to have all the patch points on the bottom so they do best on the top row, vs Mother-32 or E370 have all their patch points on the far right, so they do better on the far left of a case
some personal principles I also consider, though they might not work for everyone:
- choosing carefully what belongs on the edge vs in the middle. e.g. personally I prefer modules without screens, but I find screens less annoying if they are pushed off to the edge of the case. I prefer having I/O modules on the edges (pretty universal), as well as things like the Ears, so I can thumb the contact mic without cables or anything getting in my way (less universal for some reason). OTOH I love having my Expert Sleepers modules dead center since they are typically the master modulators of my system, and might be modulating other things anywhere in the case.
- no ugly modules; life’s too short. not to say I’m aiming for the prettiest rack possible, but I don’t want anything in there which I personally find so ugly as to be uninspiring, no matter how popular or useful or even great-sounding it might be
Plus of course it has to have the right mix of module types for your musical goals. Tbh that’s way too many constraints for a normal brain to think about, so the approach I think works is just to be aware of all the many ways of thinking about it, and then to just pick a select few which you care about and ignore the rest. And be aware you can’t meet them all at once, so just do the best you can to balance between them. Anyway here’s my main system. It’s kind of a blend of the ideas I mentioned.