Plan 9 certainly has some cool ideas, but I have a really hard time believing that a project that originated in the 80s and has yet to "make it" in any meaningful sense is really the way of the future.
But I'd really love to have an OS that sweeps the pile of legacy cruft we've built up and replaces it with something simpler & smaller. In a sense Plan 9 is just that. But I'm sure one can go further down that road.
Union mounts have been particularly interesting: something plan9 implemented, but no one else has successfully.