Piano. I've decided that more programming languages or web stacks are a complete waste of time. Piano is for life and I wished I had started doing this in earnest long ago.
My repository for this (code finished, just need to add photos and add a write up about how to use):
https://github.com/whyboris/Digital-Piano-LED
I added a feature that the left and middle pedal buttons navigate through sheet music (PDF left/right button). And I'm also running Pianoteq which makes any (even dinky) digital piano sound like a $100k grand piano (or any piano you pick for that matter) https://www.modartt.com/pianoteq
Still WIP, but easy to edit: https://github.com/whyboris/Digital-Piano-Visualization
I used Processing because of how easy it is to use and how performant it is for smooth animations.
Even so, you are right, it is definitely not a growth market by any stretch of the imagination, it takes a ton of effort to learn how to play and in a world full of distractions and ready made music more than you could ever listen to in a lifetime the enthusiasm for doing this is waning. There are exceptions to this though, for instance, in Asia where playing the piano is a status thing and still in its ascendancy phase.
I have this pipeline where I take a youtube video, download it as an MP3, convert that to MIDI and then edit that midi to split left/right and to ensure it is all set to time and of the proper length so I can show it as sheet music.
I knew next to nothing about music theory when I started this beyond the very first basic stuff, but after watching many youtube videos on the subject and reading a lot of stuff it is starting to make sense. This is a subject that is overcomplicated to the point that it seems much harder than it really is, there are only very few good teachers out there to make that which looks complicated but it ultimately relatively simple simple again.
If you want I can do a write-up of all the resources that I've used over the last year and a half.
Currently practicing Ennio Morricone's "Chi Mai" arranged for piano.
Progress is still slow but if I compare my sightreading spead, accuracy and general quality of play with a year ago the difference is huge even if you can't see it day-by-day. Each new piece brings new challenges, and teaches me something that re-inforces the pieces that I already know how to play which then all get a little bit better.
Overall I'm having a ton of fun with this and the joy I get from playing a piece end-to-end without mistakes, at speed and in a way that is nice to listen is hard to describe.
It's obvious that an experienced pianist would probably laugh at the level of my accomplishments but that's fine with me, I'm enjoying this and that's what counts. It's also one of very few things in my life that I've done without any commercial goal, and which is just for myself. Overall if I could do this life over I'd tone down the business career in favor of making more music. And the programming skills came in handy while making pianojacq.com, so in a way this allowed me to combine two things I love.
Will absolutely do this :) I'm so down to see my progress as I go. Today I had my class and started working on some chords!
best!