Combat would differ from world to world they did not use materia in final fantasy 1-6. or anything past 7. FF5 had a job class system 6 used magicite, 7 used materia and 8 you used the Draw command to get magic.

Classes: The definition would change from world to world. because Paladin has only been in 4 and 11 ( also FF1 if you count the knight class and FF5 if you take the Knight class and have White Mage as its sub).

Dragoon Has only been in 4 and 11 ( 5 had a lancer that had jump, 7 had cid which the materia system made you what ever you wanted him to be and Jump was only a limit break ability, Freya in 9 had jump but I cant remember if she was a actual Dragoon and Kimari in 10 had jump but as an overdrive and he was more of a blue mage any way.)

Know lets do a little cosmetic.

Full old Knight like Paladin armor in 8 which is a more modern setting. Does this make sense to any one else?

Races would be all human. Why? Well I don't see a Bangaa or Dwarf in 7 or many other FF do you?

Each Final fantasy is its own story, lore, world and art style. ( yes they may have some creatures and jobs that are the same but that is it.).

If this is not enough to show you why it would not be a good idea. Then write a Fan fiction. Because I have seen many good reason on why this would not work on the is thread.

This is not a smoothie you can not blend different Final Fantasies to gather and have something yummy.