I do not know exactly how current adventure system works, but I guess it's similar to the easter egg event- U get a token, click on it and do something on a new map.
How about making players send ships towards those islands? There could be a bunch new and special buildings to support building various ships from wood and metals (transport troops, attack/ convoy defence, trade, fishing ships), a dock, shipyard etc.
On new maps players could fight with each other, build/ occupy fortresses (using troops from mainland) that control bigger areas than current outposts, gain bonusses for their clan by doing tasks (like control a structure for a period of time) or just hunt treasures with their generals.
I mean- since we all live on some kind of a rectangular island, it would be logical and more e... moody to travel by ships. It would add a lot to the game too.