Yoshi already stated years ago he will not be featuring characters appearing as crossovers in FFXIV whose 'stories are already complete' or 'who are dead by the end of their respective games':

Originally Posted by
Gamerescape
Also, in regards to future collaborations with other FF series, we will be adhering to the below principles:
· We will never introduce characters that would destroy their meaning or story
· We will not use characters that are already dead or whose role has ended in their title
For example, “Red XIII” who appeared in FFVII.
I believe we mentioned “Red XIV” as a joke before, but Red XIII has a very rare and specific role in the world of FFVII. If he was wandering around the world of Eorzea it would destroy the image of the world of FFVII, so this can’t happen.
This is just a simple example of what we mean.
So that kind of rules Sephiroth out (1.0's final boss Nael van Darnus was technically a loose expy of Sephiroth anyway so that's another nail in the coffin for a Sephiroth battle in FFXIV).
As for the elemental fiends, which FF game are we talking about here? FFI's fiends are out of the question as two are already in the game,
Tiamat is a npc in the Heavensward storyline and who most definitely will never become an enemy - she's stated as remaining bound in Azys Lla in penance for summoning her deceased brother Bahamut as a Primal and is a benign, sympathetic character. And the Kraken is just a sea monster that is the final boss of both Hullbreaker Isle NM and Sastasha Seagrot HM dungeons.
I'm not sure about where Marlirith and Lich could be though, although the Palace of the Dead sounds likely for the later. FFIV's fiends though are possible. But we'll see in the future as the game develops anyway.
And finally you can get around the post limit by simply posting a few sentences, and then going back and editing the post afterwards with the rest of what you wanted to post (like I just did).