FFIX - Crystal World
FFXI - Ru'Aun Gardens - As we know Yoshida does like content that takes place in the sky and alluded to XI's 'Sky' as an eample.
FFXIII - Gran Pulse style fields - Seeing herds of monsters roaming around and an obvious 'lobby' field that connects to many other locations.
FFCC - Miasma type dungeons - Basically something that requires much more than just fighting, having a player or your chocobo dedicated to keeping the miasma away would make an interesting spin on dungeons granted the A.I is dumber than bricks.
FFXI - Empyreal Paradox/Empyreal Paradox - Abyssea - Basically beautiful battle arenas that encompasses an entire storyline in one final showdown.