I second the Squarp Hermod suggestion, and suggest looking at the Pyramid. It's not a rackable module, but it includes MIDI and some very interesting polyrhythm and Euclidian options. It's ridiculously powerful and I think one of the best sequencers on the market now. It has become the central hub for my system. I even use it to control the other sequencers in my rigs (I have a 24-step trigger controller, a Korg SQ-1 and a Qu-bit Bloom for randomness and polyrhythms), as well as five other hardware synths in the studio. It does it all without a single hiccup, a rather impressive feat. Have a look.
"I'll just plug this in here and see what happens."