Spoilers of The Maiden's Rhapsody FFXI, I can't believe this, XI and XIV crystals

[Cilia]

-FF1 and FF9 are linked. Necron is like an advanced final form of Chaos, then there's the 4 orbs/crystals of light (Desert Star/Falcon's Claw/Maiden's Earring etc) which are shards of Alexander-plus the 4 Fiends are there. I know FF9 was a medley of all FF themes, but FF1's were the strongest present.

Necron is a physical manifestation of death, sort of we think. Chaos is a demon lord.

The four crystals in I are elemental. In IX, they're all non-elemental (or rather Holy elemental).

The Four Fiends are there as fanservice, as IX deliberately homaged many previous franchise titles and took the series back to its European fantasy roots after VII and VIII's more modern / postmodern settings.

-FF7-X are linked. Sphere and Materia are synonymous. Orbs of energy that contain memories, whether it's a video recording, or teaches the user magic. FF7 probably takes place on a different planet from FFX maybe tens of thousands of years in the future, when Spira's machina has advanced enough for space travel or something.

This actually has sketchy canon confirmation.

-FF12 and FFT are linked. According to the FFT creator, FFT takes place thousands of years after FF12, when a great calamity destroys the advanced civilization.

This is canonically true, but I tend to leave Tactics out of things since, despite being one of the most beloved, it is a side game.

-I wonder if FF8 is linked to another FF? There's some much Lore in the game that's never fully developed. The Centra had these mobile 'shelters' which are really like Airship cities. There's also this Lunar Cry business which is never explained really, and the whole story of the Great Hyne who split his body and created the Sorceresses. Hyne was a great Mage in FF3. There's also the Tomb of the Unknown King, which is never explained either.

The lore of VIII is pretty sketchy, or rather, more deliberately spelled out in an Ultimania, but that's because the main focus of VIII was Squall and Rinoa's romance. We don't need an in-depth history lesson of VIII's world to understand that.

In III, Hein (not Hyne) was a minor villain who uprooted an important tree (whose name escapes me at the moment) and used it as a mobile fortress. The Warriors of Light offed him, so he couldn't pass his power down to future generations of women.

• The theory that the first Lunar Cry mentioned to have happened once in the past in FFVIII was actually the time in which the Crystal Pillar falls from the moon in FFIII. It's theorized that the Crystal Pillar is actually within Lunatic Pandora (Lunatic Pandora was built around it... also, gotta love that name, right?), and that it is the lit up section we can see within the structure when Adel is drawn out from the Lunar Cry. This would also explain why some of the building is mechanical, while other parts remain crystalline (as seen when you actually go into it), and it is stated in one of Laguna's flashbacks that the Estharians (??) were excavating the structure and building something around it (the one where They almost die and have to jump from a high cliff).

Nothing falls from the moon in III. The Crystal Tower is just there, unexplained, like many things in the game.

The Lunatic Pandora was built around a Crystal Pillar Esthar excavated for the purpose of making the Pillar mobile, that Sorceress Adel might use it as a weapon. Nobody's quite sure when the Pillar fell from the moon, but it can be used to trigger calamitous Lunar Cries, so the Esthar government under Laguna sank it. (Then Seifer got it back during the game's events, and... yeah.)

• Hyne is referenced several times in FFVIII as the progenitor of the sorceress line (when Rinoa is named a sorceress they refer to her as Hyne's descendant), and that the continuing theme of Gardens raising SeeD (essentially child soldiers) to fight Sorceresses is a continuation of the war against Hyne continued all the way from FFIII (which also involved children fighting against him).

True, but III's villain is named Hein, not Hyne. Semantics aside, Hein is a minor villain and was killed in the first half of III.

Gardens raising SeeD to fight Sorceresses was an idea a time-displaced Squall gave to Edea Kramer after the final boss in VIII, making a time loop.

• Battleship Island (the Deep Sea Research Facility) is in the exact same spot as the floating island in FFIII. It is assumed that it was built over top of the crashed island (unable to stay aloft without the one person who kept it that way) and that the Research being conducted there was related. The fact that Bahamut is found there serves as proof, as it is also the same place in which he is found in FFIII. His line of questioning, in which we have to give the "secret answer" for why we fight to obtain him ("Because it's our nature") is also considered to be a reflection of the events concerning children being caught in a conflict with Hyne. It basically says flat out that it's their legacy, and Bahamut agrees to it because he bore witness to it in both games.

Surprisingly, comparing the two maps between the games does show some similarity...

... but anyway. This is pure speculation. It's a possibility, but there are endless possibilities. As I've noted twice before, Hein in III was killed by the Warriors of Light about halfway through the game, and had no lasting effect on the storyline. If he were VIII's Hyne, I'd imagine he would have a greater impact on the story. He's just a flunky to make the Scholar class necessary for a fight. (Hein changes his elemental weakness often, and Scholars can identify it with ease. This is also why XIV Amon - who was based to a degree on Hein - can use every elemental spell in his fight.)

EDIT
-Hein and Hyne are spelled the same way in Japanese, so the mistake is understandable. Still, III Hein was a minor villain drunk on the power of darkness who was killed off almost immediately upon his introduction, while VIII Hyne is more or less the original god of the world (hence Sorceress' ability to use real magic instead of relying on Odine's para-magic draw system, since they have a small fragment of his essence).