For me the lack of female and male outfits from previous Final Fantasies/Squaresoft/Enix titles is a let down.

Two outfits I'd love to see in game either through loot grinding or achievement is Princess Ashelia B'Nargin Dalmasca's outfit from FFXII. And Beatrix's outfit from FFIX.

Others:
Rinoa's outfit/hair from FFVIII, Tifa's hair/outfit from FFVII. Kaldea Orchid's hair/outfit from The Bouncer.
Zell's outfit/hair from FFVIII as well- I don't mind making mock versions of these, but I'd rather just the actual look, not endless search for an item that is just barely passable as Ashelia's swallow tail jacket.
Another neat option would be the dress sphere outfits from X-2, although I don't like the game itself, I did enjoy the outfit designs and their unique appearances.
Seifer
Vanille/Fang
Edea Kramer/Sorceress Edea
Crystal Chronicles glams



Please feel free to make suggestions!