@DraNick
The link local address normally comes from the MAC, but can be changed, for example my LAN gateway address. However, what you can try is changing the MAC address, so that it will provide the correct link local address. That can be done on the WAN interface page. I suspect what you're seeing is pfSense is not allowing you to set a 2nd link local address, as only one is allowed. I don't know if there's a way to change the WAN link local address directly, as happens on the LAN side.
When a link local address is created from the MAC, fffe is inserted in the middle and the 7th bit is inverted.
Also, the link local address only has to be unique on the local connection. You could use the exact same one on another interface. For example, I have fe80::1:1 on 2 interfaces as shown below.
inet6 fe80::1:1%em0 prefixlen 64 scopeid 0x2
inet6 fe80::1:1%bge0 prefixlen 64 scopeid 0x1
The difference is the interface ID.
As I mentioned, link local addresses are often used for routing, as a router only has to know how to reach the next hop. In fact, with point to point links you don't even need any IP address. All you need is the interface that connects to the next router.