I've been wanting to get into OS dev for years now, I may make an attempt at it soon. When I was younger I built my own kernels for the early OnePlus phones. Maybe I can build an alternative to Android, doubtful but I like a challenge.
The hardest part to making an alternative is the app ecosystem, you almost need a complete suite of 3rd party apps built before you can get any initial adoption.
Didn't Google recently kill AOSP and stop providing board support packages for their phones?