As you have noticed from the world map, the Known Skies is only made up of a small portion of the Cerulean Skies. At the moment, the Holy Isles has a monopoly over pure cerulite trade. The other powers are nervous enough to want to break the Naga monopoly. The Aeronavis Company might be secretly dispatching airship drive technologies to the rest of the powers in order to break this monopoly and stagnation in growth. These voyages will chart new islands and find the strange and wondrous in these untouched frontiers. It is up to you to create these strange and wondrous things!
What kind of characters will participate in these exploratory voyages? Maybe your characters that are transported into this world will find themselves being favored by a lord, and gotten appointed as captain of an expedition? Or that your characters, having lived and stranded on this world for so long, might feel that being a member of an exploratory airship crew might offer them an escape from their predicament? Or that rumors of magical gates that are passages into different worlds noted on some ancient maps motivates them to join the expeditions to have at least a hope to get back to their realities?