You're right, any viable replacement for a filesystem must be compatible with existing filesystem APIs. That doesn't mean you can't implement the storage layer differently, or add additional APIs.
Look at ZFS. It's a filesystem, but internally looks like a database. If you try to define the terms "database" and "filesystem" generally, you'll realize how difficult it is to separate the concepts.
Look at ZFS. It's a filesystem, but internally looks like a database. If you try to define the terms "database" and "filesystem" generally, you'll realize how difficult it is to separate the concepts.