Many Final Fantasy fans are familiar with Ivalice. The world where many games are set in with a lot of lore. Tactics,Vagrant Story,FF12. Some great games in a great setting. I think they can do the same with Eos from Final Fantasy XV. No matter how you feel about FFXV it's no doubt there is a lot we didn't get in that game we could have. We never got to fully explore som areas. Some we don't get to go to at all. There is also a lot of talk of wars and kings from long ago. In fact in a DLC mission You fight Gilgamesh aka the Blade Master who speaks of great the war that happened 2000 years ago he fought in and died. He mentions the land looks different now than it did then..which is kinda expected of course but they could add on to that in another game. His character could be an important part in another game set in the past.
Maybe take Final Fantasy 16 and set it 2000 years in the past and we get to explore more of this world and areas we did in XV only they look vastly different with different towns from back then,maybe even massive castles as we see ruins everywhere in XV. They could also set a game far in the future where Noctis is just a myth/legend much like the Kings of old in XV. Now it doesn't have to be mainline. Another tactics? Another Vagrant Story like game? Anything really. Final Fantasy VII got many spinoffs on PS2,PSP and other platforms. No XV doesn't have the same love for it VII got but if XIII got 3 games then why not XV but much more broad similar to Ivalice. Last option is fo wha Trails of Cold Steel does and tell a story from people in the opposing land.