We think Manta is pretty cool but it wasn't actually an inspiration for us.
I'm not sure I fully understand your criticism of the filesystem. Pfs was actually initially conceived as almost exactly what you're describing, a small service on top of ZFS. But we realized early on that ZFS was going to be a pain because of its patent status so we used btrfs instead. I think btrfs is worse than ZFS but I'm not sure I'd call it a shitty local filesystem.