My first consideration would be ergonomics. Where will the case physically live in relation to the Matriarch? You will be patching between the two quite a bit. So I would get that worked out. It also needs to be secure. That's a lot of money in delicate electronics hitting the floor if it falls. Have a plan.
I would start with a Pam's New Workout (8HP), Disting EX (8HP), and an Ornament & Crime micro (8HP). Between those three modules you should have an excellent playground of possibilities to add to the Matriarch. They are also offer a great sampling of features you may (or may not) want to add to your system as dedicated modules like quantizers, additional LFOs, sequencers, etc. Apart from the case, you're looking way under a $1000US for all three modules and only 24HP of room.
That's how I would start exploring as an adjunct to my main axe.