I think the biggest thing I see here is that you have either 3 voices and 1 modulation module, or 4 voices and no modulation modules. Neither seems useful.
Second biggest thing: no VCAs.
Last question: how are you playing/sequencing this? That makes a difference for suggestions. If you're using midi, then not much to worry about... but if you're looking for in-rack sequencing, that would be good for us to know (though you have precious little space for it).
Assuming you want to keep this independent, I'd consider a 1u O_C, 1u VCAs, 1u noise/random, a 1u stereo mixer, and a Zadar or other quad modulation source.
-- Shakespeare
Just wanted to reply to your thoughtful comments! Completely agree that the case was pretty ridiculous with so many voices and so little plumbing and modulation. Honestly, this exercise keeps shifting as I play with it. I think I'm ending up more where Lugia is taking things below, turning this into some sort of task-specific monosynth or some sort of supplemental pallete for my other cases.
On the other hand, the past couple of days I've actually been messing with 4-channel MIDI CV in this case through a Mutable Yarns, which actually gave me a way to make use of all those voices (albeit not with much subtlety on tap, lol). Now thinking this might be a really fun way to build a portable, MIDI-keyboard controlled polysynth using part leftovers and part new gear.
Your suggestions really helped me think about where this case is going, appreciate it.
NICU on YouTube
NICUmusic on Instagram
For my baby daughter Luna who fought very hard to be here