https://www.google.com/search?q=open+soure+hardware+site%3Ag...
Are you interested in developing a particular type of hardware? Mechanical, electronical or other? You could try searching for particular file types, e.g. KiCad schematics files on Github/Gitlab for example.
Where I live I get extreme lightning storms that take out some of my electronics quite often. I've replaced the boards that control my swing gates twice. They're like $400 per board. This year I got tired of paying that and I replaced them with DIY esp8266 + relay modules running esphome. But this could be wildly improved with custom open source PCBs.
Another example: I've spent a lot of money on robotic lawn mowers. There's a lot of room for a good open-source contender in this arena too. Some of the top of the line commercial mowers use RTK for gps positioning (no more boundary wire) and try to incorporate computer vision algorithms for planning, but they're not super amazing, and they're also not very hackable. If the entire stack was open source then I am sure a big community would sprout around it and I think the open-source robot would quickly become the best available. You can leverage other open source projects like ardurover to get going pretty quickly.
If you take on either of these please let me know and I'll be a happy customer #1.
(and shameless plug: https://github.com/solokeys)
I agree, hard to find projects at the stage idea where you can contribute. I think the only way is to follow the communities and try to plug in when you see something early stage.