Hmm, I like the host-managed caching. It would be nice if it let you "pin" certain files and directories to the cache, which would essentially prioritize them (but, they'd still have to get demoted to slower storage, as the drive's capacity nears full).
As for other stuff like smart-prefetch and posting writes from less-utilized CPU cores, those are optimizations I'd rather see the OS do. That's easier for the latter, but smart-prefetch fits in with host-managed caching because you need to store additional usage data to support it, which could be tricky if it's not built right into the filesystem.
Thinking about it some more, it seems like host-managed caching could be a standard NVMe thing. It wouldn't be supported across all drives, but it seems like you could add some advisory bits in the NVMe protocol that would enable cross-vendor implementations. That would enable the host management part to be handled by the OS, as well.