I’ve only been on this train for a year so take my perspective with a grain of salt… I’ve been building my system with the thoughts of it being a contained instrument, supplimented by some other external gear like a drum machine, sequencer, etc. I, too, take a lot of inspiration from the shared system. Watching MN videos taught me a lot of general synthesis techniques, and seeing how a small set of tools provides a wide variety of options helps me when making module choices. The thing that has helped me the most is imposing constraints, the big one being case size. If I know I have a limited amount of HP then I have to choose my modules carefully to serve my needs. Then, I highlighted three of the most important aspects of synthesis to the music I intend to make: timbre, rhythm, and “playability” (i.e. minimum amount of menus, ease of improvisation).
When I make module choices I keep those three in mind always. So for example I’ll break down two modules I view as keepers. Just Friends checks all three of those boxes: polyrhythmic LFOs, multiple envelopes simultaneously, easy to use interface, drone monster. It’s limited in certain ways but provides so many potential uses and is really fun to use. On the other, the Cursus Iteritas has been my favorite oscillator in euro thus far. It only has one output, can’t be used as an LFO, and doesn’t always do well with audio rate modulation, but it does so much for me in the realm of timbre. I love the way it sounds and the controls are parameterized so well that it’s easy to interact with and change timbres in a significant yet controlled way. And of course since I have a size constraint, I have to work to strike a balance between modules that have multiple applications and modules that do one thing really well.
The last thing I’ll say is that it’s helped me a lot to keep my constraints loose and fluid. At one point I was trying to include lots of percussion in my rack as well, but found that it was spreading me too thin on space and required too much in terms of sequencing. And I was convinced at one point that I need four dedicated oscillators but also found that spread me thin on space. Having something like a filter that can be a great oscillator (3 sisters 
) but not always using it for that purpose is a good compromise. I’m a big believer in limitations inspiring creativity, which is what I think an modular instrument is better at providing over the modular studio route.
