i think there is something to be said for small cases with a well defined purpose, like effects or modulation - but beware scope creep. although that ship may already have sailed.
if you really wanted to cram is as much as possible and don't mind tiny knobs, 2hp Sine and Takaab 2lpg would fit in 4hp give you some additional bloops.
To give you something different to your mini synth voice and the subsequent, maybe 2hp Pluck? Mount it with 1hp space either side...
with 4 note polyphony, Pluck is also a useful source of chords: with Decay high and the output into a closed vca run 3 or 4 fast notes into it (trig/v-oct) and then open the vca with a short a/r envelope either alongside the last trigger (to keep a bit of the pluck sound) or shortly after (for a more classic synth stab) or maybe even a bit of both. 2hp Brst is a good partner module for this.
