Ok Here goes

The order of the later ff's can be roughly determined

ff7
ff9
ff8
ff10 ?

Now, in ff9 there is a place called Tifa on the intro map and also a villager describes a legendary hero with blond spiky hair, who wielded a great sword (cloud)
In ff8, the tutorial shows an old wizard the inventor of the card game, who shows remarkable resemblances to vivi (ff9).

This hypothesis supposes that all ff's are on the same world, the evidence for this is in the games. After FF7 ends we see a primordial world...which requires dramatic rebuilding. FF9 is a medieval world perhaps 1000 years after ff7. Then ff8 must follow ff9 due to the card reference and the apparent rebirth of technological culture.

The most solid evidence is for ff7 then ff9 followed by ff8. However, when would ff10 occur? Perhaps it follow FF8 as the Al Bhed ships components show similar design elements to ff8's garden. This factor alone is very weak...yet I doubt that ff10 would occur all the way before ff7, as this would be the only other chronological possibility.

Oh and if you believe that ff's occur in different worlds then how did a lowly villager in ff9's Ludlum speak of a legendary warrior with blond spiky hair and a large sword........explain that sceptics.