What if the next trilogy involves a time after this trilogy? What if its years later and Ascalon has already been rebuilt, Nightfall complete, Factions united? Perhaps some great threat comes into play which forces all three continents to fight against new lands planning an invasion or some such plot device. While there would be problems bringing old characters into it(depending on how far in the future it is), it would be a way to connect the old countries with a new set. It would also allow for the addition of starting any class on any continent(since now each country has since resettled into peace), and perhaps races as well. Maybe this would be more suited as a 'Guild Wars 2' than a chapter 4, as each chapter currently appears to be set in approximately the same time period.
Or what of a prequel, a return to the original Guild Wars which kept Tyria from allowing guilds to build Halls upon their lands? Again, time travel would be a problem, but think of the possibility of discovering 'lost' secrets and skills.
Perhaps this would be more appropriate in your stickie, King Symeon, but I was attempting to address a chapter, not a general expansion.
Anyway, what direction do you guys think this next chapter will take Tyria? (I'm not talking themes, I'm talking possible plots/reasons for new continents,etc)
But this is just my two cents

Please discuss yours.