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.
Bonus points if it includes other types of constraints: e.g, time it takes to make dinner, propose different recipes to avoid trips to the market.
LOTS of bonus points if it *suggests* events to get the family out of the routine: Thu is looking free? Go hike with the neighbors.