I cant imagine clearing up the Hydaelyn/Zodiark story without going to Sharlayan.
It boggles me that the Moon was given away already as obviously the final zone of the expansion, but then it's in the logo, so I guess there wasn't much choice in the matter.

I would have expected that we perhaps get a couple zones in Thavnair, a couple zones in Sharlayan, a zone in Garlemald, and then finish up on the moon. But that doesn't quite fit in with there being a final 'secret' zone.

Unless a curve ball is thrown as we get 2 Thavnair zones, 2 Garlemald zones, the Moon, and then somehow conclude the story in Sharlayan.
This could perhaps work if Zenos/Fandaniel play the role of Vauthry, and some other secret big bad ends up being the true final boss.