OR they can just make this game like the Settlers 2 and 3 series. Where you have a ship wrecked settlers on an island. That fights NPC on that island and if you dominate it you complete the map and move on to the next map or back to your island.
Simple things we can introduce to this so that it ties back to the main island.
1. Adventure Map that when you start it it gives you a new unit type (like Quartermaster) but its called "BOAT". The BOAT icon appears and allows you to transfer resources/units. Different BOATS can be different allocation allowance (for difficulty of map).
So you can allocate enough to start a storehouse, wood cutter, stone digger, etc. The bare minimum to colonize the new island.
2. From there on it plays exactly like the old settlers game. If you beat the island you must now build a new BOAT to haul the resources back to your main island.
The new BOAT can carry a limited amount of resources that you've gained on that island back. So you might want to upgrade the boat etc or wait til you harvest enough resources before the adventure time runs out? dunno. needs balancing but the idea is that if you beat the island you can decide how much of the loot goes back to your main island. Adventure is then completed.