What about something like this?
I have most of the modules suggested here, and I think you could get a lot out of them. Some of the other folks here will likely have other recommendations.
As this is a small rack, I tried to pack a lot of functionality and utilities into your rack so you won't get bored quickly. Maths is great, but you can pack a lot more into that 20hp. Disting can serve as just about anything, so it's almost a must-have in a rack this size. For Eno-esque evolving textures, you will want a lot of modulation sources so I have included Sloths for slow random movement, Quadrax for envelopes and LFOs, and Pique for a little bit of everything (envelopes, LFOs, drum sounds, mini-sequencing options, etc.). The 3xMIA attenuates CV signals, and you will also want VCAs to control volume over time.
Let us know how it goes.
Have fun!