I shouldn't think it would invalidate the idea if a larger case is called for, though. Larger case = more possibilities. Plus, case prices have been on a positive downward trend as of the past year-plus. The keys to look for are space, beefy power capacity, and price. For example, Erica has a 6U x 126hp cab with serious power (2.5A on each 12V rail, optional 5V!) for only EUR 300 right now, direct from their site.
A processing "nexus" would be a significantly different sort of build than one normally sees on here, though. You'll want to concentrate less on audio generation and more on modification and modulation sources. This isn't to say that you'll want NO oscillators, but you can stick to a few fairly simple ones for AF modulation purposes. So, that saves a little bit there.
The thing that'll need space here is sequencing, timing manipulation (logic, skippers, Euclideans, comparators, divider/multipliers, etc), and modulation that's more of a "free-run" variety, such as a Batumi + Poti or a Maths (better yet, both). The Lancet expander needs to fit, as well. The Stages is a very good choice for envelopes, but putting in something really over the top for linked envelope generation would be worth your time, too...have a look at Erogenous Tones' RADAR + BLIP. They also have a very useful VCA bank/mixer in their VC8; that should deal with ALL of the VCA needs you'll have here, really.
Then for sonic modifiers, consider some of the more "interesting" filters like Xaoc's Belgrad or Intellijel's Morgasmatron, things that give you a lot of leeway on internal architecture. A Clouds clone would be useful in this, or a Make Noise Mimeophon or Morphagene. Note that a lot of what I'm pointing out are larger modules; if you use these, this also saves money in the long run by jamming multiple functions into a large space. For example, all of the Erogenous Tones modules above are about $1000 put together...but that $1000 covers 54 hp of space. Now, if you tried to do this with smaller modules, it would get more expensive and still come out covering a sizable amount of panel space, and you'd probably wind up with less linked functionality.
Last, effects...one choice might be Intellijel's Rainmaker, which bills itself as a "delay" but which really is a far more complex affair, especially when you start plying it with modulation sources. Also, definitely get some CVable waveshaping; Tiptop's got a very good one for cheap. Jam in a stereo VCA mixer like Qu-bit's Mixology, and there you are.
So, yeah...you'll spend money. That's the nature of modular synthesis...it ain't cheap. But if you "go big" with multiple function modules, the money can get spent on some very complex architecture.