Given the contstraints and outboard FX here's a 62hp palette that I would find fun with tons of modulation and a lot of creative ways to patch
Starting from top left 1u row:
midi to get clocks, CV and gates from Syntakt
Steppy for 4 chan of step sequencing
Att (2x) to tame some CV levels
Stereo out to connect to the case outputs
3u row:
Pam's Pro Workout for clocks, LFO and all the magic of Pams
Mimetic Digitales for stepped modulation
Scales to tame CV modulation into notes
Plaits (clone) has the most sonic variety of any oscillator
FILThy looks like a good multi-mode filter and sticks with the After Later Audio theme
Stages (clone) packs a ton of features into a single module - LFO, Envelopes, Step Sequencer and even an Oscillator easter egg mode
dVCA is a good dual VCA
(preveiw is currently showing an earlier version with Maths that would also be fun - Wasp looks like it might be too deep for the case. click through for the updated rack)