The current maps we have are fairly small in many regards, but there is a means of expansion while offering more challenging content all within the existing areas.
The answer is simple, non instanced "dungeons". Right now all of the open world content available is out in the open, rarely are there any instances where you're inside a cave or a mine and even then most of those areas are incredibly small.
What the game needs is subterranean levels beneath each map. Non-instanced caverns, mines and ruins would fit perfectly into almost every area currently available in the game and these areas could provide a chance for the addition of more high level and dangerous content.
Right now the surface world is beyond tame with limited aggro, enemies that give up the chase 5 seconds later, and the ease of chocobo access no matter where you go. For the more casual players it's a virtual haven where you can goof around and have fun without having to worry about anything, but players that want their open world to have some teeth are left behind.
But with a subterranean world this can all change. You could have mounts too afraid to venture into or be called within each subterranean area, most enemies would aggro requiring players to actually think about how they move around or fight their way through. You can offer high level farming grounds perhaps with an increased drop rate or higher quality drops to compensate for the difficulty or even be able to have high end fates or boss-like monsters tucked away.
Additional space per map, an option of increased difficulty and more content for players all without actually having to add new areas to the game and maintaining an open world feeling outside of instanced dungeons all while retaining the player friendly surface world.