A few, actually:
1) a 2 hp passive mult of some sort. I like SSSR's, since they also incorporate a switch to change the module from 2-in/6-out to 1-in/7-out. Pretty much anything beyond a simpler level small skiff rig needs at least one.
2) Another VCF. Something rather straightforward, actually, such as a basic low-pass or multimode. The Polaris can do this, yes, but it's also something that can work as well (if not better) as an effects processor with its simultaneous ability to waveshape via overdrive plus operate as a phaser with user-specified modulation. You might also go a little weirder here and consider Doepfer's A-106-1, which is based on the Sallen-Key pair found in the Korg MS-20 and which also contains an insert jack pair to drop something into the resonance feedback circuit. That might also be a fun place to insert the Polaris, or to screw around with the audio in that path via the Phonogene. Definitely ups the abuse potential...
3) output stage. The A-138 exp is fine as an audio mixer at the end of the signal path, but you'll want to step your signal down to line level. Erica's Pico OUT might be a good choice as it also gives you a stereo out (useful with the Clouds which, btw, was discontinued by Mutable so you'll either need to have one already, source one from a used source or a dealer that might still have them in stock, hunt down a used one, or go with a third-party build...which isn't a bad idea, as those are usually smaller than the original while retaining all of its functionality), and it also provides a headphone amp, all in 3 hp.
Not sure about the CV->MIDI interface there...is that to lock your synth's clock up as a master with your MIDI, or for something else? If the former, there's probably methods that would work just as well outside of the case, although I would suggest having a MIDI clock (better resolution than the 1ppn clock) for your master and using a little thing such as a Bastl Klik to sync the modular to the MIDI, not the other way around. Plus, if you're talking about locking this to an Elektron Machinedrum, that also should output MIDI clock and start/stop, so a Klik would actually lock the modular to that just fine. Note that with sync, when at all possible, you want to start with a higher resolution clock and go downward to lower resolution ones, not the other way around.