Well the half way point marker on story is making people assume that each patch has the exact same amount of story development. A patch can have more story in it then others. 2.1 barely had any real story to it past introducing white acsican. 2.5 is extremely likely to be final patch of 2.0 cycle. Since sub patches are getting meater and 2.6 would have issues with coil finish, ilv increase right before player lv increase, introducing yet another new primal etc etc 2.6 is already looking at alot of problems.
SE has stated that some big event is going to close out 2.0 cycle and right now there are two major plot points and one darkhorse that can have a major effect on Erozea. Two likely ones, A: Omega weapon is activated (by who doesn't matter) B: Ascian influence on Ishguard is publicly exposed or C: new Garlean emperor is crown and empire turns it's attention back to Erozea. All there will likely happen in some form or combination but since expansion is hinted to be Ishguard centric i'm leaning towards B. A and C also might feel like to much of a retread of 2.0 if they are given too much focus.