First and foremost this is a strictly theoretical. I will be organizing my ideas through my theories but in essence this is one major theory. About me: I have played and completed every Final Fantasy more than once (some more than 4 times).
Major points i will be address are:
Cid
Dissidia
Final Fantasy IV: After Years
Final Fantasy IV: After Years Crystal
Example of the link they all share
I will be referring to the Lufians a lot so for short hand i'll call them Luf
Cid
Cid is as we've learned a Luf. Point blank we have gained this information from FF1. Cid is the creator of the airship and never visibly seen throughout any of the actual Final Fantasy. Though it would be an almost valid assumption to say the Cid from the others may be descendants from the actual Cid.
Final Fantasy Dissidia
Final Fantasy Dissidia seems to be the prequel to Final Fantasy. Crystal in hand looking over what were lead to believe is Castle Cornellia, the Warrior of light sets out to his world as all his friends leave. In Dissidia the timeline is an endless one where Cosmos and Chaos pick their warriors to fight. Why are they pitted in this battle is unknown until Duodecim had a cutscene between Cid and Cosmos. Cid purposely holds this battle but for what reason we do not know.
Final Fantasy IV: After Years
Final Fantasy IV After Years takes place after Final Fantasy IV (duh!) Long story short the world is queued for destruction. By who? We find out at the end of the game that the "Creator" is responsible for this.
Final Fantasy IV: After Years Crystals
While traversing the depth in FFIVAY you come to fight many familiar faces. Fun little tidbits Square Exix put in such as Death Gaze Ultima Weapon Lich and Maralith. We even were given little dialogues from our heroes basically saying that they did not belong of this world. Everytime you encountered one of these was when you interacted with a crystal in which it would shatter and a monster would emerge.
How does it all link up?
Final Fantasy IV After Years and Dissidia explain a whole lot more than they let on. In fact Dissidia makes Cid seem almost like the Creator in Final Fantasy IV: After Years. In FFIVAF The Creator talks about his dying race (which i believe are the Luf). He also explains that his motive for destroying their planet was simply that they had not developed as fast as he would like.
Well what does that mean?
The Creator(Cid) Explains about how the crystals functions were to observe the world he created. If indeed that is true then the data the crystals procured from FF1 and FF6 would explain The Archfiends appearing in FF4 and would also explain why each game is given a number
The Number Theory:
Each FF is numbered by its release (duh!) But lets use the example that the creator gave us. He observes these worlds searching for the perfect one. He never interferes what so ever. So to say that would mean that when playing a Final Fantasy game no matter what you choose the games outcome will remain constant. So its almost as if the player of Final Fantasy is actually playing as Cid. We fight in RPG style watching over our party. And through careful observation(which is dumbly obvious) you would notice how technology steady increases per game (Final fantasy 1 had 1 Airship Final Fantasy 13 had artificial deities.)
The Endless Battle example:
If all my information is correct the timeline of a true final fantasy is a loop as such. I will use FF1 and 7 for the example
Final Fantasy 1- You fight Garland in the chaos shrine
You fight the four fiends
You go back in time and fight Chaos
Cid then makes Cosmos and Chaos choose their chosen warriors
They are once again pitted in battle(with no previous knowledge of the events that happened except their arch enemies)
If Cosmos wins then the warriors return to their lands and begin the game anew Final fantasy VII
Cloud defeats Sephiroth (AC happens) and at some point they are returned to dissidia
Cloud no longer has any recollection of the past events (hence he doesnt know Tifa is duodecim 012)
They Fight Chaos and returned back to his land
I know my delivery of the information is bad but its the best i could muster this has wrecked my brain for too many hours now xD