I was wondering about that for a while now - it feels in my last few jobs as an EM, the major part of my work (or rather the most influential one?) was managing, coaching and guiding product. The realization was actually quite simple for me: while hiring in engineering is defined by an sometimes absurd number of interviews, code challenges and so on, product is a case study and you're good: and that doesn't seem to be doing the trick.
Building Split Flap Displays. Started 18 months ago and kept me on a super interesting learning path. First shot was using open source designs (https://github.com/scottbez1/splitflap), but then kept building more and more parts myself. Coming from a software engineering background, getting into designing mechanical things – and then more importantly the electronics around it - has been really challenging, but also very rewarding. At this point I have my own screen printed flaps, custom PCB Design and a, what I consider, really smart protocol that allows me to daisy chain a basically arbitrary number of display elements. It's fun!