filesystem data syncing
The phone should be able to automatically synchronize its data (files, photos, backups, etc.) with other devices, including a computer at home or work, and removable storage.
Ideally doing so over the internet and connecting over a LAN directly with other local devices, without requiring access to any server on the internet, whenever possible (lan/wifi/usb/usb-otg).
This is not only important for the note taking app #32, but much more generally useful.
Unison https://www.cis.upenn.edu/~bcpierce/unison/ (automate-able CLI and manual GUI) is certainly the most mature and reliable file syncing solution. Checksum driven, robust against time skews and time zone changes, only transmits diffs, handles file and directory renames without re-transmissions, ..., and the latest versions also support inotify filesystem triggers.
Syncthing https://syncthing.net/ has build-in support for it's own (dns based?) dynamic IP discovery, firewall traversal and relay nodes system, but still lacks some of the mature features of unison.