First off, very smart to look into the 6U case. With discipline, that will keep you satisfied alongside the minibrute 2 for years to come. The trick is, once you have a system of modules, keeping them and actually using them. Most people (myself included) go through a phase of looking at all the newest modules coming out, and buying and selling and rebuying and reselling, chasing the “perfect” system.
This just leads to less money in your bank account and less music being made. So try very hard to focus on actually creating with your modular!
What type of production are you looking to get into? The Op-1 and Minibrute 2 alone gives you quite a range. So really it comes down to what you’re looking to add to those two amazing synths.
I’m a big fan of Low Pass Gates like the LxD, Optomix, dplpg, lifeforms dynamic impulse filter.
Or even the Endorphin.es Grand Terminal which has several different types of filters and gates and effects built in.
Most people will tell you to get a Make Noise MATHS becaue it does so many things.
Buy modules that jump out to you, and don’t forget about simple utility modules like Links from Mutable Instruments, or mults, and switches stuff like that really makes a system go a long way.
what do you want to do with the modular? process sounds? add more voices?
6u is a good size case. 88hp per row gets tight quick.
have you tried VCVrack.com? you can get a decent feel for some of the modulars like clouds, rings, plaits, stages, marbles, plus a million others. also you can get an idea what utility mods you might want to add down the road.
def keep an eye on the 2nd hand Eurorack market, some good deals out there depending
this could be fun rig with the op1 in mind. you could probably find a micro version of clouds to save a few hp.
@AudioJack absolutely do try VCVRack. Every case ends up uniquely fingerprinting its owner, so VCVRack is a free or cheap way to figure out what kind of modules you would like or need. One route you take could be sequencing and using various modules to program said sequences - e.g. logic, routers, sequencers, random generators. Another could be powerful complex modulation to give you evolving and interesting timbres within single or stacked voices. Modulargrid is your next stop for planning your space. 6U/88hp is a smart choice. 3U/104hp needs extreme focus, so ends up being not large enough for a beginner. You could also go for a hybrid setup and get an ES-8 module to connect to a laptop, or the FH-2 for USB-hosting your OP1 (!)