To have a MMO based on stories of the Main series of Final Fantasy is horrible.

If you take stories of a Main title and make it a story in the mmo there is no surprises. We played them all we know the bosses, the twist , OMG did you see that moments. We know the story. You would also be taking away from the characters of that world. in a MMO your the hero as in your created character which means the main heroes of that final fantasy would be gimped.

Your getting in to a bad territory of even crossing final fantasy games with other final fantasy games.

One may like 7 while the others may like 12.

And who really wants to see a Spirits Within story line.

Also talking about how the races are generic. There the same as 11 just reskinned and different lore behind them.

You can also argue the FF1 had elves and FF1 and 4 had dwarves which were smart little people ( I think they were smart in 4). We have materia (it doesn't work like it did in 7) and magitech mounts are coming sometime in the future ( just like the ones Terra, Biggs and Wedge ride in 6) so they are already kind of pulling past final fantasies in.

Your idea works better for a time traveling Crono mmo then it does a final fantasy were no story ever corresponds with other unless there is the number 2 beside the roman numerals.