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Thoughtful responses senor-bling, Lugia, troux, and GarfieldModular. As often as we recommend that beginners start with a bigger rack, that same advice applies to those of us with larger setups. I definitely wish I had invested considerably more in extra rack space to begin with, even though my original plan was fairly large. When I was first starting to build my modular, I didn't like seeing empty space between the modules. Unfortunately that led to a few impulse purchases that didn't work out for me. As we all know, impulse eurorack purchases can be expensive and a waste of time (I'm looking at you, Loquelic Iteritas Percido...). I probably should have invested in more empty rack space instead of more modules.
As far as mission-specific rack builds, I have thought about putting together a 104hp live skiff (inspired by @troux). That would be a fun and difficult exercise. I've also considered a small skiff of modules that I want to spend more time learning. For instance, I have kept Phonogene and Clouds in my rack because I see their value, but I rarely patch anything into them and haven't discovered their subtleties yet.
I remember Mowse's excellent layout post. Great advice on module grouping. As the rack continues to grow, it will be important to keep those grouping and signal-flow ideas in mind in the planning stages, while still taking into account our personal preferences and evolving music and individual workflows.
Thanks for the responses!