If you make all the final fantasies the same art style it loses its identity. I will use Final Fantasy 9 as an example. 9 is chibi and that is what some people love or hate about it. take the chibi away and its not 9 any more it is something else sure it may have Zidane, Vivi and Garnet (Dagger). But it loses some of its charm.

So you want original stories to stories that have already been told. The world may be established but like I said earlier in a MMO you are the hero. So your character will overshadow the heroes of that final fantasy or they will overshadow you.

And that's why they made After years or 10 and 13-2.