to add to what @ferranadsr has said

I recently bought a MOOG Mavis to test the eurorrack waters and got inspired to take the plunge. Given my love for this thing, I'm planning to build the rest of my system around it, but within some serious constraints. I want this to be a fairly minimal system for financial and simplicity reasons, so I'm planning to get a 4u palette case (not negotiable)

I think this is utter folly, why? - see below - but if you absolutely must only have 4u - then get the biggest version there is - the 104hp... personally I'd suggest bigger, still... you will need it eventually and eventually is often much sooner than you think... remember, there is absolutely no need whatsover to fill a case - there are always blank panels and there are always cereal boxes (or similar) that can substitute for blank panels... the tiny cases are great for influencers to showcase individual modules with limited patching options and they're great as satellite, mission specific cases - sequencer, control surface etc, but as main synth cases - not so much...

Considering the constraints, I've included a pedal i/o to connect it with what will most likely be a MOOD effects pedal for some thicc reverb and looping potential. https://www.chasebliss.com/mood-mkii I've also secured a deal for a BIA and Noise Tools 1U from Craig's. Figured Steppy is a must-have for this system too, so the whole bottom rack is kind of locked in (ignoring the Mavis probably vacating).

if you wan BIA to do all your percussion for you - ie you want to use it as a percussion factory - I seriously recommend a jasmine and olive traffic module - it will allow you to program changes in the sounds of BIA...

I'm not convinced steppy is the going to do exactly what you want - please describe exactly what you envisage doing with steppy... it generates triggers - 4 of them, with only 2 places to send them...

BIA - I don't own one, but just from looking at it I can tell, that it massively benefits from modulation, something you don't have a lot of (other than some random from noise tools)... and modulation sources benefit from attenuation, attenuversion/offset... that you don't have any of if you are using the duatt as a mixer, which you will be - as you don't have any other mixer in the rack...

the effect pedal is stereo, yet you can only send/return a mono signal... is this a constraint you find acceptable? if not the you need a different interface and a way to take your mono sound sources and place them in the stereo field...

I've never built a system before and I still don't understand a lot of the utility of different components utilities like VCAs, although I hear they're pretty hot. That being said, I'm really looking for feedback about how this system could work as designed, what could/should be changed, and what the longer term potential is with whatever components that might eventually backfill the MOOG's 44hp. Ideally though, I'd like to create some music like I described with the Mavis in place for now, but please let me know if that's unrealistic. Thank you!
-- natural_causes

Seriously I'd do some research on the fundamentals of synthesis before commiting any money... ie what's a VCA, what's a filter, what's a wavefolder etc etc - the pinned topics at the top of the moduwiggler 1u& 3u subforum are a very good place to start...

NOW The Hard Part:

I don't think that you're going to be able to achieve what you want from these modules... I'm assuming that you're going to want to use the BIA as a one stop percussion factory... ok no problem - as long as you have something to change the 'program' on BIA in a purposeful way... but you haven't - see my recommendation of the Traffic module above (this is effectively what it's designed for - you can program in voltage changes that allow you to get repeatable prgoram changes on modules that can benefit from it BIA and Plaits, for instance)...

your mixing solution is poor... how are you going to mix the outputs of the BIA, the Mavis AND the pedal return... what are you going to send to the pedal, just the Mavis? just the BIA? and how are you going to listen? just on headphones or a 'laptop expansion speaker'? are you happy with mono only? see above for - modulation that needs attenuating...

Some further wisdom - from years of experience with modulars - buy a much bigger case than you think you'll need to start with... it will save you money in the long run... larger cases are cheaper per hp than tiny cases... you will get GAS & you will need it... not only this, but just taking the example of the Traffic module... there are modules you think you want and there are modules that you haven't even started to think about that you will need in order to get the most out of the modules you think you want... and to some extent the modules you want can end up being miserable without the ones you need...

take a look at my signature... think about it, think about it deeply... it's years of experience condensed into a few words... especially the formula... which is a quick guide to how to get the most versatility in patching, from the least cash...

ignore the case size for now and spend some time working out what you think you'll want (ie more than just a single voice and a percussion factory) in say a years, maybe twos, time and what you will need in order to actually get the modules to work together & then get that sanity checked... & then find and buy - or build the case on that including at least 20-30% expansion room (you will need it - estimate power consumption for unfilled space at 10mA per rail per hp) and then allow 25-30% for overhead - to work out how much power you actually need... and then get a case that fulfills those needs - hp and power...

in other words build the case around the modules that you think you will want and those that you will need in order to support them - don't try cramming modules into a tiny case - ergonomics will be shit and/or functionality will be poor - leading to a miserable experience... which can only be remedied by either spending money or selling up!

a very large proportion of people who ignore this advice buy another, bigger case within 6-12 months of buying their 1st case...

this is exactly what I did... I started with a tiny case (6u/72hp) & within 6 months it was full so I bought another bigger case, and then a few months later I started building a case - it was shoddy and consisted of planks of wood held together by blank panels and modules, but it worked - & I got better at building cases... years later I have over 1800hp of cases, mostly filled... but if I'd bought the bigger case to start with I think there's a good chance I wouldn't have gone past that - except for the discovery of video modules - which is another story entirely

Now I'm not suggesting that you spend your life savings in one go - I'm suggesting that you think ahead a bit, probably slightly. further ahead than next week or next month though... if you desperately want a BIA buy it now... they were recently discontinued, due to the chips used in them being end of lifed...

but do realise that in starting this you are probably going to be commiting a large portion of your disposable income for years to come to this - ie your current methods of entertainment: beer, drugs, transvestite hookers, cult memberships (whatever, I don't care & neither should you - they're only examples) will take a backseat... that 100 (insert local currency) is a module, not n beers, for example!!

"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