https://www.sqlite.org/fasterthanfs.html They're selling it quite well here, though integration into other products might be the hard thing.
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.