I'm sure most of you know by now:
GBA Additions
New Magicite:
Popular Final Fantasy series characters Gilgamesh and Leviathan can be summoned into battle
Extra Dungeon
Search underneath the earth with thee parties! But a formidable enemy awaits you there...
Monster Gauntlet Floor
Series of battles involving all monsters and bosses fought to this point. Set a new record for the party you've raised!
Reconstruction of Interface for the GBA
Redesigned windows, menus, etc. based on FF4A's construction
Rearranged, 8 Channel Background Music
New character portrait graphics, improved battle music
Newly added Monster compendium, Music Player
As of now it's slated for a winter release date.
Lately, FF VI has been kind of boring to me, but as a former FF VI addict, I'm sure this will get me going on it again, especially with the addition of two new espers, and possibly new magic. (The addition of a new element, probably water, to black magic has been stated as well).