I was actually building something similar to Juice using S3 as an object store and optionally using redis(fast) or s3(slow) for metadata storage. Basically a log structured filesystem using rolling hash chunk encoding and delegations. I kinda stopped when I found juice (and to some extent seaweed) as they were much further along. If you need shared storage and don't have crazy performance requirements it makes a lot of sense to separate out metadata and just throw blobs into object storage.