Quad Mixer/VCA would be my choice, given the space constraint. Adding a couple more linear-only DC-coupled VCAs for processing CVs is a good idea as well, plus these can be found in small sizes. As for the EG/LFO, I think you might get a little more in just 2 hp more by going with Intellijel's Quadra; the expander would be a little much to squeeze in, but the basic module alone gives you loads to work with, and two more than the Erica to boot. Plus, if you do go with a quad VCA/mixer, having the Quadra would set you right up with envelope control over each VCA.
Buffered mult? Naah...you're not going to be running so many devices on a CV line that you need to worry about voltage sag. Those things are for total lunatics (like myself!) that run a dozen or so VCOs and happen to want to stack every single one on the same CV line.
Another VCO? Loads to choose from in that general size range, with the STO being a pretty good pick among them. Of course, you could always try to shoehorn in something a bit bigger, and if so, my pick would be Doepfer's rather new A-110-6...TZFM in 12 hp for $250. But either/or there...STO gets you into the weird harmonic tilt area, A-110-6 gives you FM weirdness, either's a great pairing with the Braids. Or hey...split the difference, go to 10 hp and look at Mannequins' Mangrove. But yeah, adding one would be a good idea; the only issue then becomes what VCO to choose, and there's LOADS to pick from! Damn shame, all this potential sonic abundance...
Last, the Verb. Thanks to the mutants who're concocting the 2hp stuff, you can now cram both a stereo digital reverb AND a delay line with CV into the same 4 hp, and since these aren't things you might be apt to grab all the time, the tiny control size probably wouldn't be an issue. However, if it was, just park the 2hp modules in between a couple of others with more 'real estate' and then you should be able to get finger access pretty smoothly. Still, with the Erica, reverb seems to already be well in-hand...maybe a 2hp Freez looper could work? Or more perversely, they also make a Turing machine that was apparently left in the dryer too long, and connecting that shrunk-down thing with the RYK-185 (Metropolis) could get...interesting.
Don't sell these teensy-weensy modules short...they can make even as little as 8 hp of open space get pretty action-packed.
