Hi iben,
I asked the same question 2 years ago, and got some great advice from some power users and a suggested setup that really helped direct my rack planning: https://www.modulargrid.net/forum/posts/index/10559.
One of the pieces of advice I got was simply, "nothing about this screams generative to me - how are you expecting to generate random sequences? and modify them over time?"
Part of doing generative music (to me, this is opinion, after all) is using the modules to set up the conditions, restraints, and rules that direct and filter generated randomness into something evolving and (controllably) unpredictable. Unfortunately, doing so often takes a lot of support modules, and with a Rackbrute, there isn't much space to get generative. So my thoughts are that you should be looking at:
1) Sources of randomness. Maybe I'm wrong, but it doesn't seem like you have enough ability to create random voltage sequences. PAMs can do this well in several flavors (random and smooth random), but I find that those 8 outputs get used up fast for other clocking duties. I really enjoy my TipTop/Buchla Source of Uncertainty, but it is regrettably huge. Instruo just put out a new Tagh, which gives multiple types of random in 4 hp. @Sweelinck's suggestion of Turing Machine or Wogglebug are also great. However, maybe consider the uOrnament and Crime, which has different shift register/turing machine appletts, along with a couple of dozen other utility uses. And which brings us to Point 2.
2) Utilities to ontrol the randomness. Well, obviously someone was going to say utilities. But there are a few in particular that you might want to consider that really help me when doing generative.
* Comparators - These send out voltages when a certain threshold is met, or in the case of window comparators, when a voltage is within/without a certain range. For example, sending an LFO through and having gates come out when the LFO crosses a certain threshold. I love Joranalogue's Compare 2 for this, and the DivKid video on it goes into depth about how comparators work in general.
* Sample and hold - Gosh I love these. I'll often send a smooth random signal into one of these and sample at rhythmic intervals to develop my melodies. There are a ton of great ones, but I've really been enjoying After Later Audio's SHTH, which gives you both sample and hold and track and hold, and a few other goodies, in 2hp.
* Attenuators/attenuverters/inverters - My god, a random voltage generator really means random, and that often sounds like crap. So you'll want to attenuate the randomness, or the LFO, or the other cv, or frankly any signal because I just can't think of much that an attenuator/attenuverter/inverter can't help with. PAMS can do this, as well, but again with the limited outputs. A matric mixer like the Doepfer A-138m can be large, but quite useful. I like the Happy Nerding 3x MIA quite a lot, but getting to zero can be tricky on an attenuverter as compared to a pure attenuator. Frap Tools 321 gives you a lot of other functions, as well, but can attenuate and invert nicely. Additionally, you can also use VCAs as attenuators, if you have VCAs to spare...
* VCAS - Guess what, you don't have VCA's to spare. I find that I always want at least one more VCA than whatever number I currently have. A cascading VCA is great, but there are plenty of small VCAs out there to toss in where you have spare hp.
* Quantizer - Perhaps the Varigate can do this? I am not too familiar with it. However, if not, you'll want to be able to quantize your random voltages, because as stated earlier, randomness can sound like crap if not tamed. There are several good ones out there, but to be honest, I've not tried them other than the Dual Quantizer applet on uO_C hemispheres. But that works like a charm. I've heard people love Intellijel's Scales, which is relatively small.
3) A few honorable mentions:
* LFOs - something like Instruo/DivKid's OCHD gives a lot of organic motion in 8 related triangle LFOs.
* Mults - If you're going to be sending multiple copies of a 1v/oct signal, you will want to use Buffered or Active mults, instead of passive ones. The Frap Tools 333 is one such that provides a ton of secret uses. Doepfer's Swtched Multiple A-182-1 (passive, but amazing) is really cool, too.
* uOrnaments and Crime or Disting Super EX - A mutli-utility module that will give you access to a ton of functionality you don't otherwise have room for. Menu diving can be an issue, but they're worth the hp and scrolling in a small system.
* Mutes, routers, shifters, and switches - Don't forget that even with generative music, you'll want to play your instrument. I really enjoy fiddling with DivKid's Mutes, vpme.de's T43 Frequency shifter, and Instruo's tain and (1)F for these purposes.
Hope that helps, best of luck!
(Edited to add that I forgot PAMS can quantize signals as well, if needed, and for spelling, since I apparently need more coffee.)