Relatedly, it would be pretty cool to be able to build your own filesystem features as layers on each other (such as block-level compression or encryption, block-level forward-error correction, cache promote, striping or other redundancy etc.). A modular FS with pluggable (but also upgradable) components and some sort of structured feature-flag schema metadata, and a way to migrate between variations (such as switching to a different compression or encryption algorithm).
You may also be interested in FreeBSD's GEOM subsystem for storage.
File systems aren't relational databases, but more like NoSQL half a lifetime before it was cool. They're typically backed by the same data structures and solve the same problems.