I do think all the stories are related somehow. There are many things that just seem to appear out of nowhere (the magic for the Searing, for example), and I feel as though at somepoint all of this will be brought together (maybe not in C3, but in whatever the last chapter is) and we will meet someone who is responsible for everything: the one who corrupted the vizier, shiro, and the 'corrupt ruler' in Nightfall; the one who manipulated the fortune teller and Glint, the one who caused the Searing, the Jade Wind, and the Nightfall (albeit indirectly).
I believe that this person, this thing, whatever it is thats the central cause of all the mess on Tyria will show up eventually to tie up all these loose ends. I also am quite certain that when it does, I shall enjoy placing it on the pointy end of my katana. A lot

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