Here's my take. The MiniBrute 2 is capable of sending and receiving Eurorack clock pulses. I would expand it based around that functionality. I would consider three modules initially: Expert Sleepers Disting mk4, Ornaments & Crime, and Temps Utile.
All of the modules are multifunctional. This will give you a wide palette of options to augment your brute with. As you learn these three modules, you'll get a better taste for what direction you want to go with your expansion. O&C and TU are available in micro 8HP formats. That will go a long way in a cramped case. You're looking at less than $700US for the lot if you shop around.
There are tons of videos on these modules on Youtube if you choose to go that way.
The ErbeVerb is tasty. You will always want a good reverb. If you have the budget get that too.
Once you feel you've mastered these modules, you'll be in good shape to decide what's next. If you find yourself using a specific feature of one module over and over... you might want to find a dedicated version. For example the Disting and the O&C have quantizers. If you're always using a quantizer then it might be prudent to get a stand-alone module.