Hm, first of all: Thank you for taking your time and your recommendations!
I have to admit that I also feel a bit frustrated but that has absolutely nothing to do with your posts. It’s just that I did not post this either in this thread, which seems to be more about making notes in a paper and pencil workflow, nor in the minimal setup thread jason is referring to for the very reason that both, paper/pencil or iPad pro/midi controller/small modular are not within the area I see this topic in. My post, which was actually a new thread to discuss small devices, was erased and than placed here by someone unknown (the sour feeling derives from doing this anonymously without asking or even telling me about it - it took me a bit to find out what happened and started with the unpleasant notion that what I had written yesterday night seemed to be just gone).
Regarding the proposed ideas, I might have been too verbose to get understood. I would like to discuss small devices to play/sketch ideas while being in situations like on the bus/underground or in bed, where it would not be worth the effort to have multiple small things connected on the lap to get involved with before the next stop. As in: Wait, how would this melodic/harmonic idea actually sound? Okay, just take that little thingie out of the bag or even pocket and quickly try it out, save the midi if it’s nice and come back to it later.
Like pen & music paper w/ sound combined with something that at least pretends to be an actual physical keyboard (The only things I still do with the iPad after a few years of having one is looking things up and occasionally play Worms or Tangram. To me the idea of a multipurpose touch surface controller does sound much better than it actually feels in most cases).
Thank you @gimber for that link. I did not know about those Yamaha QY yet but add it to the list of devices to be discussed. I had a short look at some user feedbacks on sequencer.de which attest them to be a little cumbersome to operate (as one might expect from 90’s Yamaha devices) but therefore high in battery consumption. Also the LCD-screen might be a bit dark in some situations. Though they can be found cheap on the used market and GM sounds might have a bit of a vintage charm. The bigger ones seem to be liked by some people though. 
I have already have an Arturia Keystep and find it a bit too big to constantly carry it around ‚just in case‘. It is nice as a simple keyboard for the modular though with its easy to use CV/gate out, quick sync configuartion set-up and the arpeggiator.
What is the issue with Keith McMillen devices, @jasonw22 ? I have not heard something bad about their quality yet.
I am still most tempted by the OP-Z, mostly due to its pocket size and because it seems to be very immediate in to use. If it had a screen to get a visual overwiew and a less tedious export function it would be nearly perfect. I wonder if the iOS screen has something like a piano roll or notation view.
