Tension could build gradually over the course of time, and other actions players take in quests and stuff, like killing garlean NPC enemies randomly around Eorzea could speed up the tension buildup, or boost it by a small ammount.
Yeah is think this is good, how about random attacks from the Garleans in certain territorys or Camps?
As an example: if the Garleans arent stopped and take over a Camp or territory you cant do Quests for that Area or you get only quests to help to free that Area again, which will provoke the Garleans to attack.
Or it will also be a Skirmish like Attack or Defense.

The linkpearl is a good Idea, so everyone will know where and when the Skirmish begins.