The case upgrade makes a lot of sense as you are already invested in their 1u modules
I would want more modulation and utilities - quad cascading vca, matrix mixer, kinks, panning mixers, attenuators, sequential switch etc
I'd look at the happy nerding fx aid xl as at least one of the effects units - smallish, reasonably priced and loads of possible effects - and better modulation options than it's smaller sibling!
If you are going to add a lot of stero effects to a load of mono outputs then at least one panning mixer (panmixjr, for example) would add a lot - sub mix mono sources into a stereo field before processing with effects
How are you mixing what you have already???
"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