More, inexpensive VCAs since you'll potentially have so many voices, you'll want more for controlling intensity of modulation.
Unity mixer for mixing triggers together.
Clock dividers for triggering events at later defined times - may not need these as much with a BeatStep Pro since you can use a channel to essentially program your own.
Envelope follower for taking outside sources and converting their changes in volume/amplitude into a voltage. Plug guitar in, strike a string; the sharp attack and moderate decay can be sent to a filter's cutoff, etc.
More inexpensive noise and sample & hold modules: noise for getting texture as a sound source, but also makes a great textural modulation source. Aging tape, foggy sounds.
Sample & hold uses a clock source and an input to create stepped voltages. It's great for random modulation timed to a rhythm. You could subtly modulate the length of an envelope that's tied to a rhythm sound, providing little fluctuations in how long it rings out.