I would say that union file systems. Recently overlayfs got merged. It doesn't work like Plan9's, but it is probably impossible within POSIX constrains. You pro

I would say that union file systems. Recently overlayfs got merged. It doesn't work like Plan9's, but it is probably impossible within POSIX constrains. You probably shouldn't use overlayfs to merge /bin directories so you could get rid of PATH as it is the case in Plan9. Interesting thing is missing mmap in Plan9. Aside other things it makes most programs work with remote files quite easily. There is no separate path for local and remote files, because either way you have to read/write to them. That also makes local access slower than it can be, but that is the price for network transparency. You can hammer mmap to work with remote files, but it may have too many corner cases. That's at least how I understand it. Please correct me if I'm wrong.