I should probably also mention that even if you implement the existing NAT traversal protocols correctly, these aren't satisfying solutions. We really need that damn time machine already, the End-to-End Principle should have been an End-to-End Law.
I'd love to see more effort to encourage laypeople to use and ask for IPv6. Imagine if instead of Microsoft and Sony inventing terms like "strict NAT" vs "open NAT" for how their gaming consoles describe network issues, they had those consoles inform their users that they can't host a game because their ISP doesn't provide IPv6 connectivity.
