Hey @nickc, please add Nextcloud, it’s a privacy-focused, open-source & self-hosted file service, made in EU (Germany). Would be even cooler to have this provider feature enabled within the Local Desktop via an official Extension or a just a Custom Path Setting. I know “Export” function exists but a local backup workflow would be superb 
Three ideas 1) You could let users connect to Nextcloud via WebDAV and then enable the backups 2) via OAuth2/tokens: OAuth2 — Nextcloud latest Administration Manual latest documentation 3) Or the more universal, local-first approach (Desktop App): let the user choose a custom folder on their file system. This would immediately enable backups with sync services like Nextcloud and OneDrive, or even external/network harddrives like NAS storage (Synology, Ugreen) 
For example I installed “Nextcloud Sync” as a Cask via Homebrew (macOS) and now it’s basically natively integrated like iCloud or oneDrive. Example path for backup folder in the Extension Settings could be something like » \Users\<CurrentUserName>\Nextcloud\<CustomWPBackupFolder>
EDIT: Just saw another thread where you already explained that you are planning to rethink backup workflows. So a +1 from me. Hope this post inspires 
Cheers!