I'm curious to see where this one goes, particularly once Mjollnir sees this thread and he's like, MY TIME HAS COME.
I'd like to assume that every Final Fantasy is supposed to be self-contained and respect its brothers and sisters, but it wouldn't be the first time someone at SE got away with linking two numbered titles in a way that makes them no longer confined to their own respectable and complete world. Remember when Nojima and Kitase dropped the bomb that VII and X take place in the same world? Would Tanaka have the brass to change something that big?
SPOILER:
Yes.
2010
Disclaimer:
Tanaka breathes and bleeds Final Fantasy. I'm not implying otherwise.
I just like joking about how badly it went over when they changed the formula just for the sake of it.
EDIT:
I think elements of the story have been preserved, but not in a way that makes the canons one and the same. I think Cloud of Darkness might be controlling Xandes, I think Xandes might be the red-masked mage that we see involved in all this ascian business, and I'm still curious to see if the ascians fit into the cabal of mages red herring in any way or if they're the Warriors of Dark, so to speak... but the same world? I'd be ... something more than shocked. Ironically, I bet following this line of logic will lead to a lot of semi-accurate guesses as to what's going on, but that big picture they're still different worlds.