There was an interview with Yoshi P the other day where he clearly indicated that he wasn't done yet, and implied there's at least two more expansions worth of Stuff he's got in mind. My theory is that there'll be two more expansions as he said, and then they'll start working on the next online FF game, probably / hopefully with Yoshi at the helm again - they've got a formula that works very well atm (I mean heck it's the second biggest MMO out there atm), and they can continue it in a next game if they choose to. OTOH if this one keeps being good throughout the next two expansions (which is what, four years? ish?), no need to build another one, which would just mostly steal players from XIV and if it's bad, would make a lot of people angry and they'd stop playing both XIV and the next one.
er anyway, here's my theory re: next expansions. 3rd expansion will take place in a completely different location that hasn't even been hinted at anywhere yet; random forest with Viera would be a candidate there. More like a filler expansion, but we'll probably see where that goes once story patches hit in the 'Sblood expansion in the next months / year.
4th expansion will become the culmination of the storyline set forth in ARR, where the clouds in the map around the Garlean empire are lifted and that area will become available, with the storyline focused on defeating the emperor and co.
After that, it's a mystery; again, I'm thinking that's when Yoshi P will move on to the next MMO project, and XIV will go into maintenance mode; there will be (content) patches and shit, but I'm pretty sure there won't be much more. That, or they'll start a wholly new storyline; I'm sure they've got the creativity to add to the existing story from there on out. Or it'll go the WoW direction and just add something that might please a demographic they haven't appealed to yet,