1. Building an open source bicycle computer. It's been over 20 years since they arrived at the scene, and there still isn't one that you could hack! An outrage! No published sources yet, I'm getting stuck on not having much experience building physical things.
2. Getting rid of the directory hierarchy. I have 10K photos, 30K emails, 10000km of GPS tracks, and 10 years of chat logs. Why can't I find anything among them? I own a computer, after all. I have 10 folders called some variation of photos/Cologne/2020/flowers! Having to organize them myself is tedious and a fool's errand, so I'm leaning towards using a database as a file system, to let me just query for files. Geo queries using a map? Yes please. Selecting bounds on a timeline? Oh yeah!
Turns out I'm not alone, Microsoft tried this with WinFS, and failed. But the idea lives on: https://www.nayuki.io/page/designing-better-file-organizatio...
3. Writing. I hope I can find the time to expand on the above on my blog.