Provide a sync option which would allow to achieve similar concept as Wasabi Cloud NAS: https://wasabi.com/cloud-nas/
Such sync option would keep track of "file accessed" metadata and based on settings (e.g. 90 days) would move file from local location to cloud/S3.
Whenever file is accessed it would be stored in local FS cache. If file gets added it would end up in local FS cache and get synced automatically to remote.
On top of that we would provide FUSE mount, which would use the existing selected local FS as a cache.
User would have to use FUSE mount in order to gain access to all files / unlimited storage. They would have to be instructed to actually not use the local FS, as it would only aim as a local cache.
It would be possible to run S3Drive on a server, create FUSE mount and then share it via SMB/Webdav in which case the end users would be able to share storage. Alternatively each user could work with their local FUSE mount, but any changes could be synced online via AMQ or other queue/notify mechanism).
Some resources: