I can definitely speak to this, having gone the approach of starting from the fundamental design of the shared system and modifying it to taste, resulting in my current system:
This obviously goes beyond the kind of modification you’re talking about, but it speaks to my experience with the shared system and the modules I chose to replace, and why.
The most obvious “missing” elements of the shared system are filtering, mixing, stereo, noise, and modulation sources. Though I consider it’s focus and restraint part of its success as an instrument- having formerly had a larger system that did absolutely everything, I found it very paralyzing to creativity.
In a CV-bus-less system such as yours, I would highly recommend putting a couple mult modules in the 1U row, because the shared system is relatively quite low on modulation sources, and being able to fan out single sources to several destinations is absolutely vital in my opinion. (not just a single multing with a stackcable, and when you start stacking multiple stackcables things get uncomfortable and bad for the jacks.)
Filtering is highly preference-based, as part of the charm of the shared system is the buchla-style lack of straightforward filters. I however really like to use filters (not just low pass) and have found the QPAS to have vastly widened the variety of sounds I can get.
Mimeophon is maybe my favorite Make Noise module yet, and worth a consideration, certainly as it relates to replacing Echophon (I’ve used it and it’s fun and has tons of character, but is a big module for being a mono delay). Erbe-verb is one I wish I had room for in the case, and one worth keeping for sure.
When I had moddemix I found myself using it almost always for VCO ring mod bell sounds, and it’s really great at that. When I got the Furthurr in place of the DPO, I scrapped it, as the Furthurr has ring mod built-in.
Added DPLPG because I’m a sucker for vactrols.
Added Autodyne because you can do some really cool things with heavy compression and side-chaining, especially with sampled material from Morphagene- can really elevate a sound.
X-Pan has been a really great addition for me so far, as I love voltage-controlled crossfading and panning, and having it also act as a simple stereo mixer is really powerful and compensates for the shared systems lack of mixing capabilities beyond optomix.
Crow is the latest experiment, to fill the gaps mostly in random little utilities I may find myself needing in a patch that I can code up if need be- sequential switch, shift register, stuff like that.