Hello Alex
Technically speaking, there’s no rule that says that you can’t use the global unicast address as the next-hop IP. However, in the very definition of OSPFv3, it states that the link-local addresses are used for next-hop purposes. Indeed, all OSPF communication (with the exception of the case of a virtual link) takes place using the link-local addresses. Consequently, only link-local addresses are used as next hop IPs for OSPF.
Now is there a way to determine the global unicast IP address of the next-hop router from its local-link address? Unfortunately no. You can have the same link-local address for every next-hop router because the link-local addresses are only locally significant per network segment.
So a next-hop is always specified using an exit interface, and the link-local address, thus making it unique.
I hope this has been helpful!
Laz