I realize the case is small. My thought was for expansion to happen via a second daisy-chained case. I'll also admit to being eager to get started. My thought process was, for better or worse, that I could get my hands on a few good pieces, learn what I've got, what I lack, and then build out the second case based on that. ;)

I do most of my musical stuff on what people would call "the budget" end of things, so not sure I'll ever end up with the rigs you guys have, and that's okay. I'm more interested in setting up an exploratory tool for playing with raw sound. My creative output thrives on limits and finding ways around those to find unorthodox methodologies and sounds. This is a supplemental tool to stimulate the creative process, and not necessarily for the creation of an entirety of a performance, if that makes sense. At risk of sounding pretentious. ;) :p

OK so a mantis costs a bit more than the tiny 'budget case' that you got but it has over 3 times as much space - so in reality it is the "budget option" - just a slightly more long term budget option - a bit like buying a jumbo pack of dishwasher tablets or whatever...

and a mantis on it's own is still quite a small modular & there's no need to fill it in one go... etc etc... and still very portable if that's an issue for you... work out the cost/hp and you'll see what I mean

I'd spend some time figuring out what modules you actually want and those that you need to support them, before buying another case - build the case around the modular instead of constraining the modular to the case... otherwise you end up serially buying small cases - which ends up more expensive than the slightly bigger case

I get that I need a filter. I also had a quad mixer, more vcos, and xoh on the list. I waited on the xoh to see if I can just pad it through analog mixer.

most decent analog mixers made in the last 20 years should be able to cope with modular levels, especially when padded, I have a 15+ year old yamaha mg10 that works perfectly as an end of chain mixer for modular - but this doesn't mean that you won't want sub-mixers or matrix mixers in the rack - you probably will

If I were to, say, double the setup with a daisy-chained case, what would you say the necessary modules would be, specifically on the utility side of things?

don't get another tiny case!!! get the mantis!!!!! see above!

but: a low pass filter, a low pass gate, a waveshaper, sub-mixers, attenuators, attenuverters, matrix mixer, logic, a sequential switch, a clock divider would all be good starting points - and definitely look at doepfer and ladik for these (but make sure the tiny case is deep enough!!) - most of my utilities are doepfer

"some of the best base-level info to remember can be found in Jim's sigfile" @Lugia

Utility modules are the dull polish that makes the shiny modules actually shine!!!

sound sources < sound modifiers < modulation sources < utilities