Well there is a very vast unexplored area in Spira's continent and we know nothing of the other hemisphere of the planet.

Also, the history of Spira is kind of puzzling. Machina and modern tech stuff was already present 3,000 years ago. By the time of the Bevelle / Zanarkand war they had been using technology for 2,000 years!!! Why no space flight ? Why no exploration of the vast jungle north of Djose, east of the Moonflow ? Why no Internet, underwater cities and other wonders of technology ? It almost seems like civilization in Spira started going downhill 3,000 years ago....

Anyhow, with all those "open ends", there are a lot of possible ideas for sequels. I myself wrote one in my fanfic and have a prequel RPG (see below). It would be sweet if Square got the team again and did a prequel or sequel