A few years ago, a friend and I came up with a FF-unifying theory. Here's how it goes:

Final Fantasy is full of repeating themes, roles and events.

Advanced fallen civilizations:
Lunarians ~= Lufenians = Cetra = Zilart = Ancients = Allag, Belah'dia, Nym, etc. = Magi ~= Bevelle

Which sometimes separate themselves from the rest of the world:
Moon ~= Sky Fortress = Tu'lia = Floating Continent = Nym

Powerful conquering empires:
Baron = Palamecia = Gestahl = Shinra = Garlemald = Galbadia = Archadia = Bevelle


And other broad categories that I'm not quite awake enough to remember right now. But this repetition of arch concepts calls to mind the story of the first Final Fantasy, where the world was trapped in a 2000-year time loop. The same events are happening over and over again...but not exactly the same. It's only a closed loop if you view it in two dimensions; the cycle is constantly traveling along another axis, making the universe a time spiral. A coil, repeating but never intersecting. This puts FF1 at the end, chronologically, since the cycle is broken at the end. It also implies that, since the themes had decayed into basic archetypes by so much repetition, more uniqueness means it's closer to the point of origin, and more cleaving to archetypes means it's closer to the end, allowing for a rough timeline to be created. So 1, 3 and 9 would all be relatively close to the end. 2 and 7 would occur at similar points on the curve, but with many coils between them. And, for the following reasons, 5 and 6 would be very near each other:

Kefka (FF6) = Enuo (FF5)

A powerful wizard wielding the power of unmaking (Void), brought low by 12 heroes, and whose defeat severed ties between worlds. This description applies equally to Kefka and Enuo. This isn't to say they are one and the same; merely that their roles are nearly identical, placing them at the same point on the curve, with bare few turns between to change them. Additionally, this could mean that, whatever else Gogo is, he doesn't have to be a slider, just really old.

Of course, this looping construct could, theoretically, make hopping between "worlds" even easier -- the traveler isn't so much breaking a barrier between universes, but simply moving sideways through time. (And what exists outside the coil? Void! Ahaha, that just came to me.)

Another strong tie I like:

AVALANCHE (FFVII) = Warriors of Darkness (FFIII)

As the Ancients spoke of in FF3, there was once a time where the the world swung too far toward Light. In that time of need, the Warriors of Darkness were called upon to restore balance. Shinra threatened to destroy the world wielding the power of life itself; AVALANCHE arose to restore the natural cycle of the Lifestream. Seems a pretty direct comparison to me.

That's about all I can remember right now. I'd love to hear if anybody has thoughts on or further contributions to this.