I agree that this is super important! It does not exist for us basic users at this time. 
Your current best bet is to use Rclone to mirror your webdav data directory to another machine via rsync. Borg Backup has been scripted by some to help automate this process.
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Request to optionally mirror federated data between instances as a remotely federated share will disappear from your instance if it goes offline. There needs to be an option for these shares to be mirrored in the webui since they already will be downloaded on any clients federating them…
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IPFS, or InterPlanetaryFileSystem for Javascript, as external storage would allow Nextcloud users to mirror their data across multiple users, similar to seeding torrent files. See the external storage request here; there is a javascript implementation that could be adapted. It has been mentioned previously on GSOC 2017 and here discussing P2P file systems.
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Request to support the Zot protocol, which is developed in PHP for Hubzilla and allows users to literally mirror and migrate their accounts across infinite trusted servers. This process is called Nomadic Identity and would completely reshape our use of Nextcloud in the best way possible! A dev from that project has even discussed the possibility on this forum before!
HubZilla, an interesting selfhosted social network companion to your Nextcloud