If we're assuming 7 main trials with their horse/bird/wolf/dragon equivalent and it's FF4 based, I don't think the 4 fiends will necessarily be individual trials (though of course they could be). I think we'll see some of the fiends encountered in a similar way as the 5.0 Weapon series where some are solo instances (like Sapphire), or perhaps they appear as a dungeon boss (like Genbu).
..If the devs stuck HARD to FF4 I would expect (Hidden for spoilers?):
MSQ Series: (4; 2 leveling and 2 at 90)
-Magus Sisters (Dressed in Thavnarian garb; possibly void related)
-Some unknown related to the Lunar Beast Tribes or the Void.
-Zodiark (Assuming the 6.0 final boss)/Fandaniel (Probably FFXIV's Zeromus)
-??? (The 6.x series boss, which is unrelated to the Zodiark/Ascian story according to devs)
...as much as I'd like to be done with Zenos once and for all, I dunno how they're gonna do it...
3 "Bonus Trial" Series (Like the 4 Lords or Weapon series)
-Rubicante
-Valvalis
-Kagnazzo -or- Scarmiglione
(I think Scarmiglione will be some kind of solo/dungeon thing, but Kagnazzo could easily be manipulating Sharlayan)
But all that said I don't think they will go that hard into to copy FF4 storyline.
...and since we're speculating, wasted potential if they don't use the Crystal Palace as a PotD or similar leveling space/infinite dungeon.. assuming there is a Crystal Palace.