Dropbox server implementation. Has a great open source MacOS client, but the server implementation is needed. I would contribute financially towards such a project, as Dropbox will never be satisfied to remain reliable file sync (and I am currently a paying customer). They keep churning features to expand the TAM. This will eventually lead to enshittification. Underlying storage could be any S3 compatible target. Each file could be an object, stretch goal would be deduplication across blocks such that Dropbox’s Pocket storage system does.
https://www.maestral.app/