IPv6

Firewalls in sensible routers will drop new incoming IPv6 packets by default anways, unless you initiated the connection first.

So in effect it's just like NAT (people on the Internet can't see you without an initial connection), but without the internal IP:Port → external IP:Port translation and all the disadvantages associated with it.

But some router manufacturers treat IPv6 as a second-class citizen, which could be a problem.