I began to research this when I saw the various plot theories for Final Fantasy XIII. I have come to a conclusion that there is a chance that all the games are in fact tied to one another.
The setting of Final Fantasy 13 is framed around a futuristic society, which leads me to wonder something. Much like stars, through out the games it appears planets have life. But in more of a metaphysical sense. By this, I mean that all that will happen is based on the central life inside the planet. In 7, it was a lifestream, in 8, there were various things holding the planet together. It's more about the thing being threatened. In 9, there was 2 planets, and one threatened the others civilisation by exposing it to the life on it's own surface. In 10, it was a rift in the planet that attacked itself. This being said, it is plausible that once a civilisation dies out, they have no way to replenish the planet. Thus it dies, and the planet recreates itself.

Now, what must then occur is the planet builds a defense. It creates more intelligent creatures, while making sure no evil can overpower it. This stands to reasoning if the planet's life is substantial to that of a human. It makes itself stronger, and will eventually make stronger life.
Based on this theory, I can also say that the games do not go in historic order. 8 would have happened before 7, and 9 prior to 8. The world of Final Fantasy 3 would have happened prior to the rest, based on the medieval world setting.

In Final Fantasy 7, Sephiroth wanted to wound the planet, then proceed to "become" the planet. This would mean the planets never fully die, they just pass on ownership, if you will.
The games most likely follow this planet life concept, but only one really focused on it. The game would then become almost like a series of the unanswered questions, "What happened afterwards?" The games should then follow each other in a misset order, simply milleniums apart. Cloud and Aeris appear in Final Fantasy Tactics, which lends itself an automatic contradictory ideal, but that could have simply been based on a joke for the creators. After all, Final Fantasy has a large number of spin-offs.

Also, every game has a consistent tie to two different species. The chocobo is an obvious one, because it has become a staple of the games. But the moogle has been in almost every game up until X. The games worlds also seem to have evolutionary changes in their mechanics and societies, although keeping some species of monster. The most notable reoccurent creatures are Bahamut, Marlboro, and the Cactuar.

Thus I believe each time the planet "dies" and later reforms to a new existence. The existence becomes stronger then the last, but like all life, must fail. So no matter what, this existence is simply a stone throw away from extinction.
In addition, many times villians are closely related in premise, and I don't believe this to be any coincidence. Jenova is a planet originally, if you follow the whole idea she was the meteor. Kuja was created by a mad man, Garland, who had a alternate planet, similar to Gaia. So there is a chance Garland could have created Jenova and Kuja.
And if this is so, perhaps the planet that was the birth of Jenova and Kuja is the same, maybe it crashed into the planet of FF7. Then, this would back up the fact that Jenova had destroyed so many worlds.
Yes, I know there are holes, but this is the basis of my research as of now. I will continue with this and get more out later. Thank you.

-Thom