Thanks everyone.
The precision adder feels like a good idea. I don't necessarily plan to obsess over melody and tuning in the modular, but it still seems like a nice simple way to change stuff. Joysticks and touch interface stuff are also cool - the latter is probably more practical and intuitive to me, but something about manipulating Nerdseq CV with a joystick feels appropriate.
I won't be rushing any of this auxiliary stuff anyway - for now, just getting the Nerdseq itself will give me plenty to do and play around with. Really, I have about 3-5 voices to mess around with here at any given time and they are all well-equipped with modulation for now, so I'll probably only need the expanders (except the MIDI one, which seems like a good call) if I become a die hard fan. The video one is almost more interesting to me since we've got some video elements to play live with as well and I think it'd be charming to include images from it as we play.