This will be a few posts, because of the character limit.
If there's anything I love more than a good crossover in games, I haven't found it yet. Maybe it's functioning grappling hooks, but that's neither here, nor there.
I enjoy speculating on the possibilities of Eorzea being vaguely or remotely connected to other worlds in the franchise, but where games like IX and X allude to them with easter eggs, games like Dissidia and XIV go further.
I know for the most part, we all just say it's for nostalgia, or because Square Enix reuses old enemies and assets (nowadays out of tradition and self-reference more than anything else), but it's more fun to see similarities, and ponder on them.