I understand that previously there were some challenges preventing implementation of "set priority" in IPv6 firewall mangle. However now that IPv6 is part of the base product in ROS 7 I would hope that you can add support for "set priority" to the IPv6 firewall mangle.
Also of interest (although not as crucial for us) is IPv6 support for PPP tunnels (L2TP etc) and for the TR069 client. I imagine IPv6 support was missing from all of those modules because those would have to be coded for the possibility that a given router may or may not have the IPv6 package installed. Now that the IPv6 package is always installed, I hope it is possible to add IPv6 support for these features more easily.