Many cloud storage providers allows you to create a sharing folder which allows you in return to share files with friends and family. I would like to see more features to make Syncthing used as a way to share files between others, here is what I would like to suggest.
Create public Sync ID Instead of sharing your device ID, to be able to create another Device ID or be able to have a secondary device ID that is meant to be used when sharing files with others and using your main device ID for your own personal devices
Allow/Block Subfolders An option to not allow subfolders to be synced within the syncthing folder
Data Cap Set Data Caps to prevent someone from transferring a large file over. I been told this is a feature but I do not see this as an option when I edit a folder in Syncthing-Gtk.
Folder Size Cap Have a cap on how much the folder can contain, this can prevent your folder from ending up to be gigabytes in size. I been told this is a feature but I do not see this as an option when I edit a folder in Syncthing-Gtk.
Approve Transfer Instead of setting up the device and then the folder connection. You can set a folder to approve the files from another device to be sent over before they are. Not to approve weather your folder will sync with another folder, but to receive files when they detect changes. Maybe even seeing what the files and folders are and how big the files are before they are synced/transferred.
Once Time Sync Once the files are synced, it will delete the syncthing connection between the two folders.
Folder Expiration Be able to set a date and time when the syncthing connection between two folders will be deleted.
Password Require a password to connect to a device or to sync two folders.