I believe every area has a sort of "large area objective" that results in what would be considered a world boss. as making it spawn requires that a large number of objectives be completed on the entire map area within a timely fashion. not all the areas might have large sized bosses, but definitely bosses that require most of the people in the map to participate if you have any hope to kill the thing...usually.
there's tequatle in sparkfly fenn (another dragon) Claw of jormag in frostgorge sound The fire elemental of thaumanova in whatever the asura starting area was. the cursed shore has a really tough one that I missed the tail end of to see what the boss was. it's not zhaitan There's that big shade boss in the human starter area. In hirathi hinterlands there is a centaurs boss complete with 3 substantially sized war hounds. I haven't seen any other personally