I'm currently leaning towards Mutable Instruments modules, but many of them are quite large for such a limited space. There are many clones that come in smaller sizes, so I have included them. Some, however, are still quite large. Ripples, for example, is basically just a filter, but takes up 8HP of width. I'd love to hear of any alternatives in a smaller format (doesn't have to be Mutable/clone, any nice filter under 8HP).
the obvious and sane answer to this is... get a bigger case... 6u is easily portable, unless you are a small child - I've carried a mantis (best bang for buck case there is) on trains, planes and buses quite a bit and never had a problem with it being too big to carry - and then you can stay with full size modules and benefit from the ergonomics - remember 1hp is just over 5mm (1/5 of an inch) and 3u is less than 14cm (5.25") - often newbies do not realise how small eurorack actually is!!
At 8HP each, Pam's and Disting EX were included to add lots of diverse functionality in a small space. Continuing the trend of more functionality in less space, Ochd fits many spread-out LFOs into 4HP. The remaining space was filled with a handful of 2HP modules. I'm still learning about all of the functionality of Pam's and Disting EX, and they might be able to cover many functions that I might be attempting to get out of other modules, so let me know if anything seems obvious.
too many tiny (2hp) modules next to each other - they will be (virtually - at best) unusable
this rack will be horrible to play... see above bigger case, ergonomic modules
"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