NAT may go away, but stateful firewalls won't. There will still be many environments where you can't easily get a point to point connection even though the IP is addressable and there is a route.
Won’t the firewall know to let return traffic through if my computer explicitly sends a request first?
I don’t know much about IPv6 but it seems like even if we are stuck with IPv4 we could still make some sort of protocol where my computer could manipulate the NAT table entries allocated to it. Maybe something like DHCP but it deals with ports.
Isn't something like that what UPnP / zeroconf is supposed to do[1][2]?