Yes, Patrick Steinhart (GitLab) has been working not only on reftables and pluggable backends for the references data, but also pluggable backends for object storage, so that you can use any database backend format (sqlite, s3, special large file storage options, etc) to store objects if you want (in addition to loose objects and packfiles).

This is work that Patrick and GitLab have been doing for years now and it's very impressive and nearly complete.