Actually I believe Yoshi P. made mention that either the team or the players or both were kind of getting tired of the Ascian, Garlemald, Hydalyn Zodiark storylines that have been going on since 1.0 so they would be setting them all up to wind them down and finish them so they can move on to new storylines and villians. They also mentioned SB was the beginning of the end for the Garlean arch and that ShB was the same for the Ascians, and the Hydalyn/Zodiark storyline, it feels more like they are moving to conclude it all in 6.0, plus it just would not make sense to say one expansion is the leading conclusion of one storyline and then the next is the same for another but that first one has to wait two expansions to wrap up while the second one whose conclusion was started after that expansion has it wrapped up the following expansion.
I truly feel they want to wrap up both in 6.0, plus right now Islabard has the highest amount of foreshadowing. The civil war, Zenos waiting on the throne in the heart of the chaos, the towers being Garlean make and tempering people to the cause, there are even more hints toward Islabard then Sharlyan at the moment. Of course i still believe we are also getting Sharlyan, it would be kind of weird to wrap up the Hydalyn/Zodiark storyline without going there but mainly as our main hub city, with the exploration zones in Islabard. I also do not seem them doing another misdirect since they already did that with ShB and it worked there, but it will not work twice and it would get repetitive and a little annoying. Also I should point out that right about now they have done about all they can with the Garlean Empire, and I am kind of getting a little tired of facing them.
Just because they wrap up the Garlean storyline in 6.0 also does not mean the end of certain lingering threads related to them, Noah Van Gabranth and the IV legion has been set up in a way that they could continue as threats even after the fall of the Garleans, and quite frankly I find them much more interesting, their armaments' and forces are also much more diverse so it doesn't feel like fighting the same mooks and machines over again. In fact perhaps they even remain as a sort of Byzantine or the Eastern Roman Empire, which lasted for around a millennia after Rome fell to the Germanic tribes and had their own identity compared to their western counterparts.
But going back to the topic the point is I would much rather they wrap all these season 1 plotline and move on to new storylines and threats for 7.0.