I'd love to have one that randomly fails, so we could debug e.g. databases and other apps that are supposed to be resilient. I'd call it "havocfs". Doesn't corrupt data, just randomly fails reads/writes/etc. Some software is supposed to handle that gracefully. Would be a great test tool...

Devices rather than filesystems, but .. https://libguestfs.org/nbdkit-error-filter.1.html & https://libguestfs.org/nbdkit-evil-filter.1.html