1) Syncthing "middleware": don't just sync files between devices, but specify logic of what should happen when a device "sends" or "receives" the file.
E.g: NAS stores music files in high-res MP3 or FLAC, but you want (part of?) them in your phone as lower-res format.
It would be a good project for me to use and learn more of Go, and it would work well with what I had in mind when writing https://raphael.lullis.net/thinking-heads-are-not-in-the-clouds/
Just too big of a project for me to do it now.
2) A *family* calendar that can generate day/week schedules based on the constraints it is given.
E.g: Mom has meetings at this and that time. Dad on other days. Kids need to be picked at school between 3 and 3:30pm. Jr has karate at Weds.
Now, who-takes-who-when?