FFI Fiends are highly unlikely, given two are already in the game (Tiamat is a benign dragon npc in confinement in Azys Lla and definitely bears the player no ill will, and Kraken, is the end boss of no less than two dungeons: Hullbreaker Isle NM and Sastasha Seagrot HM), so that just leaves Lich and Kary/Marlaris/Marliath, which, I presume will show up in some form probably eventually, but not as a full set.

Really instead, I see the FFIV Fiends appearing instead. The Magus Sisters would also be great to fight as well.

But the previous FF summon I would love to appear in FFXIV eventually would be FFVIII's Doomtrain (no, FFVI's Phantom Train in Omega: Sigmascape does not count, as it's something completely different! ). And SE themselves have long hinted at it in multiple locations, so, fingers crossed!