In each expansion you'll probably do another set of specialty throwback fights/raids/dungeons as part of the sidequests. We had ff5 in gilgamesh, ff3 in CT, and now ff6 in Azys Lla; I've thought of ways to implement other FF classics as well.

FF7:
Target: Jenova and Sephiroth.
Story Basis: A member of a cult of nald thal, a BLM, has been desperately trying to resurrect his mother using forbidden magicks. As he taps further into the void, he corrupts both himself and his mother's remains. In a quest to stop him you follow him to multiple regions where, at each failed resurrection site you find pieces of his mother so corrupted they have taken on a life of their own as powerful voidsent. When you finally catch up to him, he is so corrupted that he becomes a voidsent himself. Only his mother's head or heart remain. Final fight is a battle against both his corrupted form, a voidsent aether swordsman, and the final tortured embodiment of his mother's final remains.