Without trying to steal the thread, what I would care is actually the opposite direction.
Neither in Swift Playgrounds, nor in any other programming development environment apps for both mobile OSes, have I found a good development experience using pen instead of keyboard.
Given how many of us "program" in paper notebooks, it is quite incredible that besides a couple of research projects done by PhD students, no one cares to actually make it more widespread in a usable way.
In the same way that the calculator writes in the answers in your handwriting, it could write in snippets of code in your handwriting.
If it's going to be made though, it has to be e-ink. Go big or go home!
I'm on this route myself, trying various things out at https://github.com/fenollp/reMarkable-tools
Handwriting (in and out) support is very important IMO. Also being able to draw DAGs.
I'd like an e-ink device with high frame rate and HW powerful enough to run some models locally or with good enough connectivity and sensors that e.g. Computer Vision tasks can be offloaded to the users' smartphone.
Feel free to expose your ideas on there :) I welcome Open Source discussion!