The only core Final Fantasy game I can see getting ANY kind of a Prequel trilogy, lore-wise, is Final Fantasy V. It just feels like... EVERYTHING HAPPENED in Final Fantasy V before the story actually started, and having it be a prequel trilogy would work out, since there's a lot of backstory there. What with the events of 1000 years prior to the story with the fighting and inevitable banishment of the necromancer Enuo to The Void and the world splitting in two, then 20-30 years prior to the story with the Dawn Warriors Galuf, Xezat, Kelger, and Dorgann with the birth and eventual sealing of Exdeath, and maybe something in between those two events, maybe involving Gilgamesh or the Demons of the Void?

A Final Fantasy I prequel would be utterly redundant, a Final Fantasy II or III prequel would be interesting, but unnecessary, a Final Fantasy IV prequel would just be milking the franchise more, a Final Fantasy VI prequel wouldn't make very much sense, A Final Fantasy VII prequel has literally been done at least twice (Before Crisis / Crisis Core), A Final Fantasy VIII prequel would be redundant, a Final Fantasy IX prequel, as cool as that would be, would be silly, I could see a Final Fantasy X prequel happening with Jecht, Auron, and Braska, but nothing that would be trilogy-worthy, FFXI would never get a prequel trilogy, I doubt FFXII would get a prequel trilogy, and if FFXIII gets a prequel trilogy I'm going to be pissed.