Hi,
I’d add one or two mults, a “rack internal” mixer or two (Doepfer mini stereo a-138s or Intellijel mixup for instance) and maybe some straightforward doepfer adsr or something like that. The MI modules are often very versatile in the env/vca field but I like to have a few smaller simple dedicated of these as well.
I’m all for the niftycase and its very functional midi/cv functions. I started out (for the second time) with Rackbrute 6U and a 4ms pod but I’m thinking of getting a niftycase too for more midi/cv possibilities.
If you’re still into keeping this relatively small I’d recommend MI Marbles for semi random sequencing and pattern creation. Marbles is just great, so musical and a real hub in my setup.
(Edit: removed unnecessary SQ-1 recommendation since you already have the Keystep :-) . )