There is no difference of opening a port in a firewall or a NAT (well, as long as there is a single machine that wants to listen on that port, at least).

And some kind of UPnP will still be required even if your internal network were using ISP-assigned IPv6 addresses for protocols that want to open multiple connections, like VoIP conferencing, bit torrent etc.

Of course, if you have an internal network where you actually communicate between your various machines, you won't want to use ISP-assigned publicly routable IPs, since those can change at any time, so you'll also need some kind of network address translation at the edge.