I've played several of the final fantasies over the years, and while they seem to have a few common elements, there seems to be some significant differences between the various worlds. Is there any commonality to the back stories of the final fantasies, or do they all take place in essentially different universes?
Entries in the Final Fantasy series share a lot of themes and elements (Cid, chocobos, crystals), but most occur in different universes. Aside from the obvious sequels (e.g. X and X-2):
- Most games let you explore the entire world; there's nothing in the way of shared geography.
- The different games have vastly different technology levels, and presence of and systems of magic.
- The different games have different sentient and non-sentient species with different pre-game histories, often explored all the way back to the origin of the planet.
There are two exceptions. First, Ivalice, the setting of XII, the Tactics games, and Vagrant Story. Second, the Crystal Chronicles series contains several games in different genres, and all occur in the same universe.
This is about it. They have common themes, character names, and mechanics.