XII and XIII already have confirmed locations people... Also, FFIII's stage in Dissidia is the Crystal Tower that also goes to the World of Darkness in Chaos mode.

My picks...

FFI - The Cornelia Bridge, it's one of the most iconic moments in the first game so why not make a battle field out of it since we never got to appreciate it much in the original.

FFII - Cyclone, I just think it would be cool fighting through the fortress and having the Cyclone in the background.

FFIII - Hard to come up with, but I'm going to say the Ancient Tree, just cause it would look snazzy and different.

FFIV - Tower of Babil baby

FFV - ExDeath's Castle, its chaos mode would be slick as it changed from normal castle to its nightmare fuel version. Though I do like the idea of the Big Bridge as well...

FFVI - It looks like its going to be the Phantom Train but I would have preferred Narshe instead. It just would have looked cool.

FFVII - Midgard, either outside the Shin-Ra building, or somewhere in the slums, hell even one of the reactors would be fun.

FFVIII - Lunatic Pandora or Edea's Parade

FFIX - Either somewhere on Terra or I would love to see Alexandria

FFX - Luca stadium, or Zanarkand

FFXI - going by the major themes of the dungeons chosen thus far in the actual game, XI's dungeon will be the place you fight the final boss, the question will be from which expansion?

Other dungeons I would like to see that don't have to be from any specific entry:

Crystal Room
Airship
Chocobo Forest
Developer's Room