Okay, so here are some things to discuss.
Does Final Fantasy have any kind of Timeline?
Airships have existed since II or III, chocobos have existed since III, I think, and moogles have existed since V (thinking again). Most monsters you meet in newer installments have already appeared in nearly every previous. The legend of the Crystals (or at least the power of crystals, which was still reflected in VII & IX) has been a running theme somewhat.
I know only of Kain and Cid Highwind, although a character named Cid usually plays an integral role in the story whereas Biggs and Wedge are usually granted a simple mention.
What is the world of Final Fantasy?
We know the name of the world in X is called Spira, several people including Yuna speak the name is direct reference to the planet (whereas in VII, it was just called "the planet." Lame).
There have been times when I've thought Final Fantasy actually encompasses tales from two or three different planets. I've personally assumed that the Fayth of X was somehow related to the Lifestream of VII. Both are the concentration of spiritual energy possessed of deceased people, animals, and even plantlife. Both grant unmeasurable power in the form of summoned creatures, though the Mako of VII being a crystallized form of the Lifestream further concentrates it's power which, I believe, is why it allows you to use magic. It'd be simple enough to say that stealing the Lifestream interrupts the flow of energy and therefore makes it impossible to cast spells (or even use heightened skills) unless you are holding a piece of materia.
What's your opinion?