I wouldn't put anything in the whole - I'd get the next case first - so there are no constraints on hp

Haha well, quite. But for various reason that next case won't be coming along any time soon. (Although I already am thinking about putting the Tetrapad into one of those little 4ms skiffs, along with its expander and a "proper" trigger sequencer.)

But, if I wanted CV mixing in a case this size I'd be tempted to replace the doepfer LPG with a MengQi DPLG and a DC coupled quad cascading vca (so you have a voltage controlled CV Mixer)

I wouldn't trade the Doepfer LPG for the MengQi one, although that is a cool little module. The Doepfer is also a very fun and controllable filter which I use often. However I think if/when I expand the setup this quad VCA idea sounds quite promising.