I was reading recently about how Tailscale makes peer-to-peer connections work, which I thought was quite interesting. If we stop using NAT there is still an issue of getting traffic through stateful firewalls. That can be hard without a server because, for example, in some cases you need to coordinate two nodes sending each other messages on the same port nearly simultaneously to get all the intervening firewalls to interpret that as an "outbound" session from both sides to allow traffic through. https://tailscale.com/blog/how-nat-traversal-works