with ipv6 there's still a firewall on the border router that will prevent incoming connections reaching your local machine's random ports

this is entirely a client problem, even with NAT there's no reason the IRC client couldn't do DCC using modern style hole punching (with the IRC server mediating)

I disagree, anything that requires Peer-to-peer needs to be considered, so firewalls will always support some kind of upnp.

Else the gamers and the people who rely on voip are going to go ballistic.

both games and voip have used STUN for the past decade or so

It could be the future—unless non-permissive firewall configurations ruin it.