Support NPt with Dynamic WAN Interfaces
Currently, using Multi-WAN with IPv6 requires a static IPv6 address on all WAN connections. Mass market cable and DSL connections from most US providers do not provide static addresses. pfsense should support the user configuring a private IPv6 subnet and using NPt on both dynamic WAN connections. Without this feature, there is no way to configure IPv6 Multi-WAN using two connections from different mass-market ISPs.
Originated with a question on the pfsense subreddit: https://www.reddit.com/r/PFSENSE/comments/6oa0ol/ipv6_multi_wan_centurylink_comcast/