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I would start with a more traditional setup to start.
1) VCO's, one or two to start, analogue or digital. With more than one, you can stack and cross modulate. I would also get a noise source. Make Noise STO is great, flexible and reasonably priced.
2) Filter, many different styles, low-pass to start but there are many multi-mode filters that do lowpass/ bandpass/ highpass simultaneously. Modulation of a filter can make your system sound like two or three simultaneous instruments.
3) VCA, I second MI Veils and Intellijel Quad VCA. WMD has the Javelin which is a full ADSR with integrated VCA and separate envelope output (great 'bang for the buck').
4) Function generator, Rise/ Fall will do envelopes on a one-shot basis or will loop as an LFO. Maths is great as is Intellijel Quadrax (4 rise/ fall, but no mix and Or/ Sum/ Inv).
5) Output module, L/R 1/4" Stereo out + 1/4" Headphone Jack (you need a means of listening to your results and get them out of the box if that is your intent). I have a WMD and Befaco but there are many others.
6) Additional modulation: Random generators, Touring Machine is great. Sample and hold (often good for a noise source as well).
7) FX. It may seem like a luxury to start but a nice reverb or delay can makes things sound huge and otherworldly. I second FX Aid as a start.
8) Mixers are important but you have only two signals to mix in your current setup. I use Befaco STMix for submitting as it will mix 4+1 stereo signals (or mono normal to both channels) and is DC coupled and will mix CV. There are many others.
9) General utility, Multiples (buffered? or not), attenuators (Synth Tech modules have onboard attenuation so perhaps not as big a deal to start),
Synthesis Technology E352 Cloud Terrarium probably wouldn't be my first choice to start but if you want to go that direction, what ever will keep you motivated and interested is important. Mantis case is a great start. Good luck!